BfOLOGY 
LIBRARY 


PLATE   I. 


Fig.  1  Fig.  2  Fig.  3 


I  I  ••! 


Fig.  4 


A 

COURSE 


IN 


EXPERIMENTAL  PSYCHOLOGY 


BY 


EDMUND   C.   SANFORD 

PROFESSOR  OF  EXPERIMENTAL  AND  COMPARATIVE  PSYCHOLOGY 
CLARK  UNIVERSITY 


PART  I:   SENSATION  AND  PERCEPTION 


BOSTON,  U.S.A. 

D.    C.    HEATH    &    CO.,    PUBLISHERS 
1903 


^*3^  f 
5:3 


BIOLOGY 

LIBRARY 

6 


COPYKIGHT,    1894,    1898, 

BY  EDMUND  C»  SANFOBD. 


TYPOGRAPHY  BY  C.  J.  PETEBS  &  SON,  BOSTON. 


PBESSWOBK  «*  s.  J.  PABK'  ILL  &  Co.,  BOPTON. 


PREFACE. 


THIS  collection  of  experiments  has  passed  through  sev- 
eral stages  and  grown  at  every  stage,  until  now  Part  I.  is 
many  times  larger  than  the  whole  course  as  originally 
sketched,  larger  than  any  one  is  likely  to  use  in  its  en- 
tirety, and  almost  too  large  to  justify  the  title  of  a  course 
at  all.  This  I  regret ;  but  I  take  comfort  in  the  thought 
that  it  will  at  least  be  easier  for  those  who  use  the  book 
to  select  what  they  need  from  it  than  from  the  sources 
from  which  it  has  been  gathered.  AVhat  a  good  laboratory 
course  ought  to  include  is  not  yet  wholly  clear,  and  can 
only  be  settled  by  trial ;  till  this  has  been  done  a  superflu- 
ous liberty  of  selection  may  not  be  wholly  a  disadvantage. 

The  experiments  cover  but  a  part  of  the  experimental 
field,  chiefly  that  of  sensation  and  perception,  though  it  is 
evidently  impossible  to  take  out  any  sort  of  mental  phe- 
nomenon for  entirely  independent  examination.  In  a  sub- 
sequent volume,  if  I  am  able  to  prepare  it,  I  shall  hope 
to  widen  the  course  by  chapters  on  voluntary  movement, 
memory,  attention,  emotion,  and  other  complicated  mental 
states,  in  so  far  as  they  are  open  to  experiments  of  mode- 
rate difficulty. 

To  some  who  turn  its  pages  the  book  may  seem  rather 
a  physiology  of  the  special  senses  than  a  psychology  of 
sensation  and  perception.  This  is  not  strange  perhaps; 
but  it  should  be  remembered  that  the  distinction  between 
the  two  is  not  in  the  experiments  themselves,  but  in  the 

iii 


IV  PREFACE. 

perspective  in  which  they  are  viewed.  Whether  or  not 
the  course  is  one  in  psychology  or  physiology  must  there- 
fore be  left  in  large  measure  to  the  user  of  it  himself.  In 
making  the  collection,  I  have  tried  to  keep  the  line  sharp 
in  my  own  mind  between  those  experiments  that  have  a 
distinct  psychological  bearing,  and  those  that  do  not ;  and 
while  a  considerable  number  of  the  latter  have  been  in- 
cluded, they  have  been  those  that  furnish  data  for  other 
experiments,  or  are  otherwise  useful  to  the  main  purpose  of 
the  course.  I  trust  that  in  this  subordinate  position  they 
will  not  seem  out  of  place. 

Most  of  the  experiments  are  demonstrations!  in  char- 
acter, and  aimed  at  qualitative  rather  than  quantitative 
results,  even  when  for  convenience  they  have  been  given 
a  quantitative  form.  Precautions  necessary  for  results  of 
the  latter  sort  have  therefore  been  lightly  touched  upon. 
The  setting  of  the  experiments  is  generally  the  simplest, 
and  the  apparatus  the  least  expensive,  that  promised  satis- 
factory results.  That  no  mistakes  have  been  made,  even 
from  these  points  of  view,  is  more  than  I  can  hope.  I 
have  been  careful  in  the  selection,  however,  and  let  the 
book  pass  from  my  hands  with  the  hope  that  it  may  prove 
helpful  both  to  those  who  have  psychological  courses  to 
give  and  to  those  who  shall  by  and  by  supplant  it  by  a 
better  one. 

A  few  explanations  are  necessary  with  regard  to  particu- 
lar portions  of  the  book.  The  first  six  chapters  remain, 
with  the  exception  of  insignificant  changes,  as  they  stood 
in  the  set  of  "  advance  sheets  "  printed  in  1894.  The 
literature  from  which  they  are  drawn  is  therefore  not  later 
than  the  fall  of  1893.  I  am  sorry  not  to  have  brought 
them  up  to  the  date  of  the  last  three  (the  end  of  1890), 
but  to  do  so  would  have  greatly  delayed  the  completion  of 
the  work,  if  it  had  not  prevented  it  altogether.  The  litera- 


PREFACE.  V 

ture  that  has  since  appeared  —  some  of  it  very  important 
—  can  be  followed  in  the  excellent  bibliographies  of  the 
Psychological  Review,  Annee  Psychologique,  and  Zeltschrift 
fur  Psychologie.  A  few  points  in  the  first  six  chapters  had 
been  marked  for  revision  when  opportunity  offered.  The 
more  important  of  these  have  been  taken  up  in  the  section 
of  notes  following  the  appendices  (pp.  433  ff.).  In  three 
instances  (pp.  16,  24,  and  27)  forward  references  to  litera- 
ture were  made  from  earlier  chapters  to  Chapter  VIII. ; 
but  when  Chapter  VIII.  came  to  be  written,  its  plan  was 
somewhat  changed,  and  these  references  must  therefore 
be  cancelled. 

The  bibliographies  include  for  the  most  part  books  and 
articles  consulted  in  the  preparation  of  the  experiments. 
A  few,  however,  have  been  merely  scanned,  and  a  very  few 
have  been  taken  at  second  hand.  The  purpose  and  scope 
of  the  bibliographies  have  been  indicated  more  fully  in  the 
introduction  to  the  first  of  them,  p.  20. 

It  has  been  my  intention  by  the  references  following  the 
experiments  to  make  full  acknowledgment  of  my  obliga- 
tion to  those  from  whom  I  have  derived  them.  I  may  say 
here  in  addition,  that,  in  the  sense  of  wholly  new  experi- 
ments, there  is  hardly  anything  original  in  the  book.  My 
part  in  it  has  been  one  of  selection  and  adaptation,  and  in 
the  nature  of  the  case  could  hardly  have  been  anything 
else.  Where  obligations  have  been  so  great  and  to  so 
many,  it  is  difficult  to  choose  for  special  mention,  but 
mine  have  been  very  great  to  Helmholtz,  Hering,  Aubert, 
Wundt,  Stumpf,  and  Goldscheider ;  and  if  what  I  say  of 
the  psych ophy sic  methods  is  compared  with  Kiilpe's  sec- 
tions on  the  same  subjects,  it  will  be  seen  that  I  have  taken 
advantage  of  his  discriminating  treatment  of  them.  A  good 
many  of  the  diagrams  used  have  of  necessity  been  taken 
from  the  sources  from  which  the  experiments  themselves 


vi  PREFACE. 

have  been  drawn,  and  are  covered  by  the  same  references. 
For  the  loan  of  the  blocks  for  several  of  the  illusions  in 
Chapter  VII.,  I  am  indebted  to  the  courtesy  of  Messrs. 
Charles  Scribner's  Sons  and  to  Professor  Joseph  Jastrow  ; 
and  for  two  of  those  showing  disks  for  Weber's  law,  to  Dr. 
August  Kirschmann  of  the  University  of  Toronto. 

In  a  less  tangible,  but  no  less  real  way,  I  have  been 
assisted  by  many  of  my  colleagues  in  Clark  University 
and  elsewhere,  and  by  many  of  my  students,  and  here 
make  grateful  acknowledgment  of  the  obligation.  This  is 
true  especially  with  reference  to  President  Hall,  in  whose 
lectures  and  seminary  at  Baltimore  the  study  of  several 
of  the  topics  of  this  course  was  begun,  and  whose  inspira- 
tion and  encouragement  have  had  much  to  do  with  its 
completion. 

In  the  proof-reading  and  indexing  I  have  been  assisted 
by  still  others,  whose  unfailing  helpfulness  in  other  ways 
makes  it  seem  strange  to  select  this  alone  for  mention. 

E.  C.  S. 
CLARK  UNIVERSITY,  December,  1897. 


TABLE    OF   CONTENTS. 


CHAPTER  I.                                         PAGE 
THE  DERMAL  SENSES 1 

Sensations  of  contact  —  Sensations  of  temperature — Sensa- 
tions of  pressure  —  General  sensations :  tickle  and  pain. 

CHAPTER   II. 
KlN^ESTHETIC   AND   STATIC    SENSES 25 

Muscle  sense  —  Innervation  sense  —  Sensations  of  motion, 
Joint  sensations  —  Sensations  of  resistance  —  Bilateral  asym- 
metries of  position  and  motion  —  Recognition  of  the  position 
of  the  body  as  a  whole  —  Sensations  of  rotation  —  Sensations 
of  progressive  motion. 

CHAPTER  III. 
SENSATIONS  OF  TASTE  AND  SMELL 47 

CHAPTER  IV. 
SENSATIONS  OF  HEARING 54 

Sounds  in  general  —  Single  and  successive  tones  —  Simulta- 
neous tones  —  Binaural  audition  and  the  location  of  sounds. 

CHAPTER   V. 
THE  MECHANISM  OF  THE  EYE  AND  VISION  IN  GENERAL     .     89 

The  retinal  image  and  accommodation  —  Entoptic  appear- 
ances —  Retinal  fatigue  and  adaptation  —  After-images  —  Move- 
ments of  the  eyes. 

CHAPTER   VI. 

SENSATIONS  OF  LIGHT  AND  COLOR 131 

Light  and  color  in  general  —  Color  mixing  —  Contrast  —  Some 
phenomena  of  rotating  disks — Binocular  phenomena  of  light 
and  color. 

Vii 


Vin  CONTENTS. 

CHAPTER  VII.                                     PAGE 
VISUAL  PERCEPTION  OF  SPACE  AND  MOTION 184 

Monocular  perception  of  space  —  Geometrical  illusions  — 
Equivocal  figures  —  Binocular  perception  of  space  —  Visual 
perception  of  movement  —  Similarity  and  symmetry. 

CHAPTER  VIII. 
WEBER'S  LAW  AND  THE  PSYCHOPHYSIC  METHODS  ....  333 

CHAPTER  IX. 
SUGGESTIONS  ON  APPARATUS 363 

APPENDIX  I. 

LISTING'S  LAW  IN  THE  HEMISPHERICAL  AND  PLANE  FIELDS 

OF  REGARD 421 

APPENDIX  II. 
SOME  SIMPLE  CASES  OF  THE  MATHEMATICAL  HOROPTER      .  427 

NOTES. 
NOTES  AND  SUGGESTIONS  ON  CHAPTERS  I.-VI 433 

INDEXES. 

INDEX  TO  AUTHORS 437 

INDEX  OF  SUBJECTS  .  441 


LABORATORY  COURSE  IN  PSYCHOLOGY. 


CHAPTER    I. 

The  Dermal  Senses. 

THE  sense  organs  of  the  skin  give  us  besides  pain,  tick- 
ling, shudder,  and  the  like,  the  more  special  sensations  of 
contact,  heat,  cold,  and  pressure.  All  these  may  be  received 
passively  when  our  members  are  at  rest,  or  actively  when 
our  members  are  in  motion,  in  which  case  special  sensations 
of  motion  are  blended  with  those  just  mentioned.  We  also 
assign  to  each  sensation  a  more  or  less  exact  location.  To 
examine  some  of  these  skin  sensations  is  the  purpose  of  this 
chapter.1 

SENSATIONS  OF  CONTACT. 

1.  The  Location  of  Touches.  Touch  yourself  in  several 
places  with  the  same  object,  and  analyze  out,  as  far  as  you 
can,  the  particular  quality  of  the  sensation  by  which  you 
recognize  the  place  touched.  This  quality  of  a  sensation  is 
known  as  its  "  Local  Sign." 

Lotze,2  A,  328  ff.,  405  ff.  ;  B,  39  ff.     Stumpf. 

1  As  a  general  term  for  perceptions  of  touch  in  the  widest  sense,  Max  Dessoir 
(p.  242)  suggests  Haptics  us  an  analogue  of  Optics  and  Acoustics.    This  he  further 
divides  into  Contact-sense  (including  a,  pure  contact,  and  b,  pressure)  and  Psela- 
phesia,  from  ^TjAd4>T?<ri?,  touching,  handling  (including  a,  active  touch,  and  6, 
"  muscle  sense  "). 

2  For  full  titles  of  books  and  articles  referred  to,  see  the  bibliography  at  the 
end  of  the  chapter.    When  several  articles  from  one  author  are  given,  they  have 
been  lettered  A,  B,  C,  etc.,  and  the  references  marked  accordingly. 

i 


2  LABORATORY  COURSE  IN  PSYCHOLOGY.  [4 

2.  Location  of  Touches.     Cause  the  subject  to  close  his 
eyes ;  touch  him  on  the  fore-arm  with  a  pencil  point ;  and 
require  him  to  touch  the  same  point  with  another  pencil 
immediately  afterward.      Estimate  the  error  in  millimetres 
and  average  the  results  for  a  number  of  trials,  noting  the 
direction  of  error,  if  it  is  constant.     The  subject  must  be 
allowed  to  correct  his  placing  of  the  pencil  if  not  satisfied 
with  it  on  first  contact. 

3.  Aristotle's  Experiment.     Cross  the  middle  finger  over 

the  first  in  such  a  way  as  to  bring  the 
tip  of  the  middle  finger  on  the  thumb 
side  of  the  first  finger.  Insert  between 
the  two  a  pea  or  other  small  object.  A 
more  or  less  distinct  sensation  of  two 
objects  will  result,  especially  when  the 
fingers  are  moved.  Some  experimenters 
may  find  the  illusion  more  marked  when 
the  pea  is  rolled  about  on  the  surface  of 
the  table  with  the  crossed  fingers,  or 
when  the  third  and  little  fingers  are  used 
instead  of  the  first  and  middle  fingers. 
Aristotle,  Hoppe,  James,  II.,  86-87. 

4.  Eccentric  Projection  of  Touches.     Close  the  eyes,  and 
tap  with  the  tip  of  a  cane  on  the  floor,  or,  better  still,  on 
the  walls  and  floor  near  a  corner  of  the  room.     Notice  that 
the  origin  of  the  sensations  seems  to  be  the  tip  of  the  cane 
and  not  the  fingers  or  the  arm.     Attention  to  these  parts, 
however,  will  show  the  true  place  of  origin.     If  the  cane  is 
held  rigidly  at  the  lower  end,  there  is  little  or  no  tendency 
to  shift  the  sensations  from  the  fingers  and  arm,  unless  the 
cane  is  limber.     The  eccentric  projection  of  touches  is  only 
a  special  case  of  their  location,  and  follows  the  same  general 
laws.     See  also  Ex.  41. 

Weber,  483  f.;  James,  II.,  31-43,  195-197;  Dessoir,  219-232. 


6]  THE  DERMAL  SENSES.  3 

5.  Judgments  of  Motion  on  the  Skin.     a.   Let  the  sub- 
ject close  his  eyes.    Kest  a  pencil  point  or  the  head  of  a  pin 
gently  on  his  fore-arm  and  move  it  slowly  and  evenly  up  or 
down  the  arm.     Require  him  to  indicate  his  earliest  judg- 
ment of  the  direction.     If  the  experiment  is  carefully  made, 
the  fact  of  motion  will  be  perceived  before  its  direction. 

b.  Try  a  number  of  times,  estimating  the  distances  trav- 
ersed in  millimetres  and  averaging  for  the  two  directions 
separately.     It  will  probably  be  found  that  the  downward 
distances  have  been  greater  than  the  upward. 

c.  Starting  from  a  fixed  point  on  the  fore-arm,  move  the 
pencil  in  irregular  order  up,  down,  right,  or  left,  and  require 
the  subject  to  announce  the  direction  of  motion  as  before. 
Compare  the  results  found  with  those  found  in  Ex.  7. 

Hall  and  Donaldson. 

6.  Feelings  of  Double  Contact,     a.   If  two  parts  of  the 
body  of  like  temperature  are  brought  in  contact,  the  two 
sensations  do  not  blend,  but  the  part  that  moves  feels  the 
one  that  does  not ;   i.e.,  the  sensations  received  by  the  mov- 
ing part  generally  get  more  attention  and  are  externalized. 
Try  with  the  tips  of  the  thumbs  or  fingers  in  contact.    This 
general  rule,  however,  has  exceptions.     Feel  of  the  palm  of 
the  right  hand  first  with  the  ball  of  the  left  thumb  (which 
gives   results    in    accord    with   the    rule),    then   with   the 
knuckle  of  the  same  thumb  sharply  bent.     Light  tapping 
of  the  forehead  with  the  finger  w^e  feel  in  the  forehead  more 
markedly  than  in  the  finger,  though  usually  with  the  hand 
on  the  forehead  we  feel  the  forehead. 

b.  If  the  parts  are  not  of  like  temperature  that  which 
varies  most  from  the  normal  bodily  temperature  will  be  felt 
by  the  other.  Warm  the  right  hand  by  holding  it  closed 
for  a  minute  or  two  and  then  apply  it  to  the  forehead.  The 
higher  temperature  will  be  perceived  by  the  forehead,  while 


4  LABORATORY  COURSE  IN  PSYCHOLOGY.  [? 

at  the  same  time  the  hand  as  the  more  expert  touch  organ 
will  perceive  the  form  of  the  forehead.  Cool  the  right  hand 
by  holding  it  a  few  minutes  in  cold  water,  dry  it  and  apply 
it  to  the  back  of  the  left  hand.  The  right  hand  may  seem 
to  be  feeling  of  a  cold  left  hand.  In  this  case  of  course 
both  the  temperature  and  form  feelings  are  credited  to  the 
right  hand.  If  the  temperature  is  not  very  different  the 
direction  of  attention  may  dictate  which  shall  be  felt  by 
the  other. 

Weber,  556-559  ;  Dessoir,  229. 

7.  Weber's  Sensory  Circles,  a.  Find  the  least  distance 
apart  at  which  the  points  of  the  eesthesiometric  compasses 1 
can  be  recognized  as  two  when  applied  to  the  skin  of  the 
fore-arm.  Try  also  the  upper  arm,  the  back  of  the  hand, 
the  forehead,  the  finger-tip,  and  the  tip  of  the  tongue.  Be 
very  careful  to  put  both  points  on  the  skin  at  the  same  time 
and  to  bear  on  equally  with  both.  Cf.  Weber's  measure- 
ments as  given  in  the  text-books ;  also  Goldscheider's 
(quoted  by  Ladd,  p.  411). 

b.  Compare  the  distance  between  the  points  just  recog- 
nizable as  two  when  applied  lengthwise  of  the  arm  with 
that  found  when  they  are  applied  crosswise.     Compare  the 
results  found  in  a  and  b  with  those  found  in  Ex.   5,  but 
remember  that  this  compass  experiment  requires  the  dis- 
crimination of  the  points. 

c.  Give  the  points  a  slightly  less  separation  than  that 
found   for  the   fore-arm   crosswise,  and   beginning   at   the 
elbow  draw  the  points  downward  side  by  side  along  the 
arm.     They  will  at  first  appear  as  one,  later  as  two,  after 
which  they  will  appear  to  separate  as  they  descend.     Some- 
thing similar  will  be  found  on  drawing  the  points  from  side 


1  For  the  apparatus  needed  in  this  and  later  experiments,  see  the  list  and 
descriptions  in  the  chapter  on  apparatus  below. 


10]  THE  DERMAL  SENSES.  5 

to  side  across  the  face  so  that  one  shall  go  above,  the  other 
below  the  mouth. 

d.  Make  the  skin  anaesthetic  with  an  ether  spray  and  test 
the  discriminative  sensibility  as  before. 

Weber,  524-530,  536-541;  Goldscheider,  B,  70  ff.,  84  ff. 

8.  Filled    Space    is   relatively   under-estimated   on    the 
skin.     Set   up  in  a   small  wooden  rod  a  row  of   five  pins 
separated  by  intervals  of  half  an  inch,  and  in  another  two 
pins   two   inches   apart.     Apply  to  the  arm  like  the  com- 
passes above.     The  space  occupied  by  the  five   pins  will 
seem   less   than   that  between   the   two.     A   still   simpler 
way   given  by   James    is    as   follows:  Cut   one   end  of   a 
visiting  card  into  a  series  of  notches,  and  the  other  into 
one  long  notch  so  as  to  leave  two  points  as  far  apart  as 
the   outer   points   at   the   other   end,   but  separated  by  an 
empty  interval.     Apply  to  the  skin  as  before.     This  illu- 
sion, though  very   clear  for  some  experimenters,  does  not 
seem   equally  so  for    all,    and   some   have   difficulty  with 
it. 

James,  II.,  141,  footnote. 

9.  Active  Touch  is  far  more  discriminating  than   mere 
contact.     Compare    the   sensations   received   from    simply 
resting  the  tip  of  the  finger  on  a  rough  covered  book  with 
those  received  when  the  finger  is  moved  and  the   surface 
«  felt  of." 

10.  The  Time  Discriminations  of  the  sense  of  contact  are 
very  delicate.     Strike  a  tuning-fork ;  touch  it  lightly,  and 
after  about  a  second  remove  the  finger  so  as  not  to  stop  the 
fork.     The  taps  of  the  fork  on  the  skin  do  not  blend  into  a 
smooth  sensation  even  when  the  vibrations  are  several  hun- 
dred a  second.     One  may  assure  himself  that  the  touching 
does  not  much  alter  the  rate  of  the  fork  by  using  another 
that  beats  with  the  first.     If  the  touching  is  carefully  done, 


b  LABORATORY   COURSE  IN  PSYCHOLOGY.  [12 

the  rate  of  the  beats  will  not  be  noticeably  altered.  (On 
beating  forks  see  Chap.  IV.)  The  roughness  may  also  be 
felt  but  not  so  strongly,  by  setting  the  stem  of  the  fork 
upon  the  skin.  The  roughness  of  the  pulses  of  air  from 
large  tuning-forks  can  also  be  felt  when  the  hand  is  brought 
near,  but  not  into  actual  contact  with  them. 
Wittich,  335  ff.;  Schwaner;  Sergi. 

11.  After-images  of  Touch.     Touch  the  skin  of  the  wrist 
lightly  with  the  point  of  a  needle,  and  notice  that  beside 
the  original  sensation,  there  is,  after  a  more  or  less-  free 
interval,  a   second   pulse   of    sensation.      The   interval  is 
brief,  a  second  or  under,  and  the  sensation  appears  to  come 
from  within.     In  quality  it  is  like  the  first,  but  without  the 
pressure  component.     The  prick  of  the  needle  point  is  not 
essential ;  the  second  sensation  can  be  observed  when  the 
head  of  a  pin  is  applied.    Too  hard  touches  must  be  avoided 
in  testing  for  these  images,  as  they  give  rise  to  a  continuous 
after-image   that   fills  the  interval.     The  second  image  is 
apparently  due  to  a  double  conduction  in  the  spinal  cord, 
and  is  therefore  different  from  the  after-images  of  the  other 
senses.     A  portion  of  the  original  excitation  is  conveyed  in 
the  posterior  columns  of  the  cord  to  the  cortex.     Another 
portion  goes  by  a  slower  path  through  the  central  gray 
matter  of  the  cord.     Of.  Ex.  32. 

Goldscheider,  7J,  168  f. 

12.  An  Interesting  Illusion  of  Length,  based  on  the  time 
during  which  a  touch  sensation  continues,  may  be  observed 
as  follows :    Require  the  subject  to  close  his  eyes.     Take  a 
piece  of  coarse  thread  a  couple  of  feet  long  and  make  a  knot 
in  the  middle  of  it.     Place  the  knot  between  the  thumb  and 
forefinger  of  the  subject,  asking   him  to   press   it   gently. 
Then  draw  the  thread  slowly  through  between  his  thumb 
and  finger  and  ask  him  to  estimate  its  length.     Repeat  the 


13]  THE  DERMAL  SENSES.  1 

process,  this  time  drawing  it  rapidly.  The  drawing  must 
not  be  too  slow  in  the  first  case  nor  too  fast  in  the  second, 
or  the  nature  of  the  illusion  may  be  suggested  to  the  sub- 
ject and  more  or  less  completely  corrected. 

Loeb,  121-122. 

For  Minimal  Contact  in  relation  to  Pressure,  see  Ex.  22  ; 
in  relation  to  Tickle,  see  Ex.  31. 

SENSATIONS  OF  TEMPERATURE. 

13.  Warm  and  Cold  Spots,  a.  Move  one  of  the  pointed 
brass  rods,  or  even  a  cool  lead-pencil,  slowly  and  lightly 
over  the  skin  of  the  back  of  the  hand.  At  certain  points 
distinct  sensations  of  cold  will  flash  out,  while  at  others  no 
temperature  sensation  will  be  perceived,  or  at  most,  only 
faint  and  diffuse  ones.  Heat  one  of  the  rods  slightly  in 
the  gas  flame  and  repeat  the  experiment.  More  care  will 
be  required  in  locating  the  warm  spots  than  the  cold  spots, 
for  their  sensations  seem  less  distinct. 

b.  On  some  convenient  portion  of  the  skin  mark  off  the 
corners  of  a  square  2  cm.  on  the  side.  Go  over  this  square 
carefully  both  lengthwise  and  crosswise  for  both  warmth  and 
cold,  drawing  the  point  along  lines  1mm.  apart,  and  note  on  a 
corresponding  square  of  millimetre  paper  the  warm  and  cold 
spots  found,  warm  spots  with  red  ink,  cold  with  black.  This 
time  the  points  should  be  heated  or  cooled  considerably  by 
placing  them  in  vessels  of  hot  or  cold  water,  and  should  be 
kept  at  an  approximately  constant  temperature  by  frequent 
change,  one  being  left  in  the  water  while  the  other  is  in  use. 
Break  the  experiment  into  a  number  of  sittings  so  as  to 
avoid  fatiguing  the  spots,  for  they  are  very  easily  fatigued. 
A  map  made  in  this  way  cannot  hope  to  represent  all  the 
spots,  but  it  will  suffice  to  show  the  permanence  of  some  of 
them  and  possibly  to  show  a  little  their  general  arrange- 
ment. When  the  map  has  been  made,  select  a  responsive 


8  LABORATORY  COURSE    IN   PSYCHOLOGY. 

and  isolated  cold  spot,  and  try  it  with  a  warm  point.  Try 
a  similar  warm  spot  with  a  cold  point. 

c.  Notice  the  very  distinct  persistence  of  the  sensations 
after  the  point  has  been  removed,  that  is,  the  temperature 
after-images. 

An  interesting  question  suggested  by  this  punctual  loca- 
tion of  temperature  sensations  is  this,  namely  :  How  does  it 
come  about  that  we  ordinarily  conceive  such  sensations  as 
continuous  over  considerable  areas. 

Blix;  Goldscheider,  A,  B,  E ;  Donaldson. 

14.  Mechanical  and  Chemical  Stimulation  of  the  Temper- 
ature Spots.1  The  temperature  spots  respond  with  their 
characteristic  sensations  to  mechanical  and  chemical  stimu- 
lation (and  some  observers  find  also,  to  electrical  stimula- 
tion), and  do  not  give  pain  when  punctured. 

a.  Choose  a  very  certainly  located  cold  spot  and  tap  it 
gently  with  a  fine  wooden  point  (not  too  soon  after  locating 
it,  if  it  has  been  fatigued  in  locating) ;  or  better,  have  an 
assistant  tap  it.     Thrust  a  needle  into  a  well-located  cold 
point.     Try  both  for  comparison  on  an  adjacent  portion  of 
the  skin. 

b.  Choose  a  convenient  area,  say,  on  the  back  of  the  hand 
or  the  temple,  and  rub  the  skin  lightly  with  a  menthol 
pencil.     After  a  little  the  sensation  of  cold  will  appear. 
Goldscheider's  tests  with  a  thermometer  applied  to  the  skin 
show  that  the  sensation  is  not  due  to  an  actual  cooling  of  it. 
The  menthol  makes  the  nerves  of  cold  at  first  hyperaes- 
thetic  (so  that  they  respond  with  their  specific  sensation  to 

1  Such  experiments  as  these  illustrate  the  Law  of  the  Specific  Energy  of 
Nerves,  which  may  be  stated  somewhat  as  follows:  Every  stimulus  that  can 
excite  a  sensory  nerve  at  all,  causes  such  sensations  as  follow  the  stimulation  of 
that  nerve  in  its  customary  way  and  only  such.  As  regards  the  interpretation  to 
be  put  on  the  phenomena  thus  generalized  there  is  dispute.  Goldscheider  1  ; 
Wundt,  3te  Aufl.  I.  332  ft".,  4te  I.  Aufl.  323;  Helmholtz,  Sensations  of  Tone,  148  ; 
Optik,  2te  Aufl.  233,  Ite  Aufl.  193;  Ladd,  307,  353. 


16]  THE  DERMAL  SENSES.  9 

mere  contact,  and  give  an  intenser  sensation  when  a  cold 
body  is  applied  than  do  adjacent  normal  portions  of  the 
skin) ;  afterward,  however,  all  the  cutaneous  nerves  become 
more  or  less  anaesthetic. 

c.  Chemical  stimulation  of  the  heat  nerves  can  be  tested 
with  C02.  Provide  two  like  vessels ;  place  them  side  by 
side  and  fill  one  with  C02.  Plunge  the  hand  into  the  vessel 
containing  the  gas,  and  for  comparison  into  the  one  contain- 
ing air.  For  the  additional  experiments  necessary  to  prove 
this  to  be  a  real  chemical  stimulation,  see  the  literature. 

Blix,  Goldscheider  A,  J5,  D,  F,  and  Donaldson;  on  c,  R.  Du 
Bois-Reymond. 

15.  The  Temperature  of  the  Skin  at  any  moment  is  a 
balance  between  its  gain  and  loss  of  heat.     Anything  that 
disturbs  that  balance,  causing  increased  gain  or  loss,  pro- 
duces  temperature   sensations.     It   is   common   experience 
that  a  piece  of  cloth,  a  bit  of  wood,  a  piece  of  metal,  all  of 
the  same  temperature  as  the  air  that  seems  indifferent  to 
the  hand,  cause  different  degrees  of  the  sensation  of  cold 
when  touched,  because  they  increase  the  loss  of  heat  by  con- 
duction in  different  degrees.     If  a  paj^er  bag  be  placed  over 
the  hand  held  upward,  a  sensation  of  warmth  is  soon  felt, 
because  of  the  decreased  loss  of  heat. 

16.  The  Shifting  of  the  "  Physiological  Zero."     a.  Pro- 
vide three  vessels  of  water,  one  at  30°  C.,  the  second  at  40°, 
the  third  at  20°.     Put  a  finger  of  one  hand  into  the  warmer 
water,  a  finger  of  the  other  into  the  cooler.     At  first  the 
usual  temperature  sensations  will  be  felt,  but  after  a  little 
they  disappear   more   or   less   completely,  because   of   the 
fatigue   of    the   corresponding  temperature   organs.     Now 
transfer  both  fingers  to  the  water  of  normal  temperature. 
It  will  seem  cool  to  the  finger  from  warmer  water  and  warm 
to  the  one  from  cooler.     This  experiment  has  been  sometimes 
regarded  as  one  of  successive  contrast. 


10          LABORATORY  COURSE  IN  PSYCHOLOGY.          [18 

b.  Hold  the  hand  for  one  minute  in  water  at  12°  C.,  then 
transfer  it  to  water  at  18°.  The  latter  will  at  first  feel 
warm,  but  after  a  time  cold  again.  The  water  at  18°  first 
causes  a  decrease  in  the  loss  of  heat  or  a  slight  gain,  but 
later  a  continued  loss. 

Weber;  Hering;  Goldscheider,  B,  32  ff. 

17.  Effect  of  Extent  of  Surface  Stimulated.     The  inten- 
sity  of    the    sensation    increases   as   the   stimulated    area 
increases.     Dip  the  right  forefinger  (or  hand)  into  hot  or 
cold  water,  observe  the  sensation,  and  immediately  insert 
the  other  forefinger  to  an  equal  depth.     Vary  the  experiment 
by  inserting  the  left  finger  first,  and  by  inserting  both  at 
once  and  then  withdrawing  one.     The  original  experiment 
of  Weber,  who  inserted  first  a  finger,  and  then  the  whole  of 
the  other  hand,  gives  striking  results,  but  has  the  fault, 
as  Goldscheider  rightly  observes,  of  adding  a  more  sensitive 
as  well  as  a  larger  area.     This   experiment  must  not  be 
inconsiderately  contrasted  with  Ex.  23. 

Weber,  553;  Goldscheider,  G,  475-476. 

18.  Temperature  Fatigue,     a.   Extreme  temperatures  fa- 
tigue the  sensory  apparatus  of  both  heat  and  cold.     Hold  a 
finger  in  water  at  45°  C.,  the  corresponding  finger  of  the 
other  hand  in  water  which  feels  neither  cold  nor  hot  (about 
32°).     After  30  seconds  dip  them  alternately  into  water  at 
10°.     The  finger  from  the  water  at  32°  will  feel  the  cold 
more  strongly.     Hold  a  finger  in  water  at  10°,  the  corre- 
sponding finger  of  the  other  hand  in  water  at  32°.     After  30 
seconds  dip  them  alternately  in  water  at  45°.     The  finger 
from  the  water  at  32°  will  feel  the  heat  more  strongly. 

b.  The  fatigue  of  the  temperature  apparatus  may  produce 
an  apparent  contradiction  of  Ex.  17.  Plunge  one  hand 
entirely  under  cold  water  and  keep  it  there  for  a  moment. 
Then  dip  the  finger  of  the  other  hand  or  the  whole  hand 


20]  THE  DERMAL  SENSES.  11 

several  times  in  the  same  water,  withdrawing  it  immediately 
each  time.  The  water  seems  colder  to  the  finger  or  hand 
which  is  only  dipped. 

Weber,  570;  Goldscheider,  B,  34  ff. 

19.  Temperature  After-images,     a.  Hold  a  cold  piece  of 
metal  on  the  forehead  or  on  the  palm  of  the  hand  for  half  a 
minute.     On  removing  it  the  sensation  of  cold  continues, 
though  the  actual  temperature  of  the  skin  is  rising.     Some- 
times fluctuations  are  observed  in  the  persisting  sensation. 
After  contact  with  a  hot  body  the  sensation  of  heat  con- 
tinues in  the  same  way,  though  the  temperature  of  the  skin 
falls.     Goldscheider  explains  this  result  for  cold  in  part  by 
the  persistence  of  the  cold  sensation  in  the  manner  of  an 
after-image,  and  in  part  by  the  lessened  sensibility  of  the 
nerves  of  heat;   a  similar  explanation   mutatis  mutandis 
holds  also  for  heat. 

b.  Intermittent  after-images,  or  those  that  recur  after  an 
interval  more  or  less  free  of  sensation,  have  been  observed 
especially  with  repeated  stimulation.  Heat  a  key  till  it  is 
just  a  little  short  of  painfully  hot,  touch  some  part  of  the 
skin,  e.g.,  the  wrist,  three  or  four  times  at  intervals  of  about 
half  a  second.  The  after-image  of  the  heat  will  appear 
several  seconds  later.  Try  the  same  for  cold,  but  use  a  key 
that  is  at  the  temperature  of  the  air. 

Cf.  Ex.  13  c.f  also  the  after-images  of  hearing  and  vision, 
Chapters  IV.  and  V.,  and  notice  that  all  the  temperature 
after-images  are  positive  ;  i.e.,  like  the  original  sensation. 

Goldscheider,  B,  11,  34  ff.,  38  ;    on  6,  Dessoir,  300. 

20.  Fineness   of   Temperature   Discrimination,     a.  Find 
what   is   the   least   perceptible    difference   in  temperature 
between  two  vessels  of  water  at  about  30°  C.,  at  about  0°, 
and  about  55°.     The  finest  discrimination  Avill  probably  be 
found  with  the  first  mentioned,  if  the  discrimination  does 


12          LABORATORY  COURSE  IN  PSYCHOLOGY.          [21 

not  prove  too  fine  at  all  these  points  to  be  measured  with 
the  thermometers  at  hand.  Use  the  same  hand  for  these 
tests,  always  dipping  it  to  the  same  depth.  It  is  better  to 
dip  the  hand  repeatedly  than  to  keep  it  in  the  water. 

b.  The  different  surfaces  of  the  body  vary  much  in  their 
sensitiveness   to   temperature.      The   mucous   surfaces   are 
quite   obtuse.     When   drinking  a  comfortably  hot   cup   of 
coffee,  dip  the  upper  lip  into  it  so  that  the  coffee  touches 
the  skin  above  the  red  part  of  the  lip,  or  dip  the  finger  into 
it;  it  will  seem  burning  hot.     Plunge  the  hand  into  water 
at   5-10°  C.     The  sensation   of   cold  will  be  strongest   at 
first  on  the  back  of  the  hand  where  the  skin  is  thin,  but  a 
little  later  will  come  out  more  strongly  in  the  palm,  where 
it  will  continue  to  be  stronger  and  may  finally  approach 
pain. 

c.  The  middle  line  of  the  body  is  less  sensitive  to  tem- 
perature than  portions   at   either   side  of  it.     Touch   the 
middle  of  the  forehead,  or  the  tip  of  the  nose,  with  a  piece 
of  warm  or  cold  metal  and  then  touc'h  several  places  to  the 
right  and  left  of  that  point. 

Fechner;  Weber,  552  if.;  Goldscheider,  B,  49  ff. 

SENSATIONS  OF  PRESSURE. 

21.  Pressure  Points.  Make  an  obtuse  but  extremely  fine 
cork  point  (pyramidal  in  shape ;  for  example,  the  pyramid  a 
quarter  of  an  inch  square  on  the  base  and  of  equal  height), 
set  it  upon  the  point  of  a  pen  or  other  convenient  holder,  or 
use  a  match  whittled  down  to  a  fine  point,  or  even  a  needle. 
Choose  an  area  on  the  fore-arm  and  test  for  its  pressure 
spots  somewhat  as  for  the  hot  and  cold  spots,  but  this  time 
set  the  cork  point  as  lightly  as  possible  on  point  after  point 
of  the  skin  instead  of  drawing  it  along.  Two  kinds  of  sen- 
sation will  be  felt ;  at  some  points  a  clear  feeling  of  contact 
with  a  sharp  point  will  be  felt,  at  others  no  feeling  at  all,  or 


23]  THE  DERMAL  SENSES.  13 

a  dull  and  vacuous  one.  The  first  are  the  pressure  points. 
Goldscheider  describes  their  sensations  on  light  contact  as 
"  delicate,"  "  lively,"  "  somewhat  tickling  ...  as  from  mov- 
ing a  hair  ;  "  on  stronger  pressure,  "  as  if  there  were  a  resist- 
ance at  that  point  in  the  skin,  which  worked  against  the 
pressure  stimulus ; "  "  as  if  a  small  hard  kernel  lay  there 
and  was  pressed  down  into  the  skin." 

The  first  are  said  to  be  more  sensitive  to  small  changes 
of  pressure,  and  though  with  sufficient  increase  both  give 
pain,  their  sensations  retain  their  characteristics.  They  are 
closer  together  than  the  temperature  spots,  and  harder  to 
locate.  The  fact  that  our  most  frequent  sensations  of  pres- 
sure are  from  surfaces  and  not  from  points  is  perhaps  the 
reason  it  is  difficult  at  first  to  recognize  a  pressure  quality 
in  these  sensations. 

Goldscheider,  B,  76  ff. 

22.  Minimal  Pressure  or  Simple  Contact.     Find  weights 
that  are  just  perceivable  on  the  volar  side  of  the  fore-arm 
and  on  the  tips  of  the  fingers.     Try  also,  if  convenient,  the 
temples,  forehead,  arid  eyelids.     In  applying  the  weights, 
see  that  they  are  brought  down  slowly  upon  the  surface  of 
the  skin,  that  they  touch  equally  at  all  points,  and  that 
their  presence  is  not  betrayed  by  motion  of  the  weight  after 
it  touches  the  skin.     This  can  be  done  by  using  a  penholder 
or  small  rod,  with  its  tip  put  through  the  ring  of  the  weight, 
for  laying  it  on.     Compare  the  relative  sensibility  found  by 
this  method  with  that  found  with  Weber's  compasses  for 
the   same  parts  (Ex.  7)  and  note   that  the  latter  requires 
discrimination,  not  mere  perception.     See  also  Exs.  29  and 
31. 

Aubert  and  Kammler ;  Bloch. 

23.  Eelation  of  Apparent  Weight  to  Area  of   Surface 
Stimulated.     Test  with  the  equal  weights  of  unequal  size 


14          LABORATORY  COURSE  IN  PSYCHOLOGY.          [24 

upon  the   hand,  properly  supported   to   exclude   "  muscle 
sense."     The  smaller  will  seem  decidedly  heavier. 

24.  Discriminative  Sensibility  for  Pressures.  Use  the 
pressure  balance  if  one  is  at  hand;  if  not,  have  the  subject 
close  his  eyes  and  lay  his  hand,  palm  upward,  on  such  a 
support  as  will  bring  his  arm  into  a  comfortable  position 
and  make  his  palm  level ;  for  example,  on  a  folded  towel 
placed  on  a  low  table  or  the  seat  of  a  chair.  (The  matter 
of  an  easy  position  for  the  subject  is  of  cardinal  importance 
in  all  psychological  experiments.)  The  method  of  experi- 
menting here  to  be  used  is  that  of  the  "Just  Observable 
Difference  "  or  "  Minimal  Change  ; "  it  may  be  applied  as 
follows :  Lay  in  the  subject's  palm  a  piece  of  thick  and 
soft  blotting-paper  just  large  enough  to  prevent  the  weight 
from  touching  the  skin.  Place  the  standard  weight  of  100 
grams  upon  the  paper  and  allow  it  to  remain  a  sufficient 
time  for  the  subject  to  get  a  clear  perception  of  its  weight. 
Then  remove  it  and  immediately  put  in  its  place  a  weight  of 
110  grams,  allowing  that  to  remain  as  long  as  the  first.  If 
the  subject  can  recognize  this  difference  easily  and  surely,  try 
him  with  109, 108,  and  so  on,  alternating  the  standard  weight 
and  a  weight  to  be  compared  with  it  till  a  weight  is  found 
that  is  just  recognizably  different  from  the  standard.  If 
110  grams  is  not  recognizably  different,  take  111,  112  in- 
stead of  109,  108.  Occasionally  follow  the  standard  with 
another  100  gram  weight  to  guard  against  illusion  on  the 
part  of  the  subject.  After  having  determined  the  just 
observably  greater  weight,  find  the  one  that  is  just  observ- 
ably lighter  in  the  same  way.  Make  a  g;:od  number  of 
determinations  of  these  just  observably  heavier  and  lighter 
weights,  sometimes  going  toward  the  standard  and  some- 
times away  from  it.  Take  the  differences  between  them  and 
the  standard  weight  and  average  the  results.  The  ratio  of 
this  average  to  the  standard  will  be  a  measure  of  the  dis- 


24]  THE  DERMAL  SENSES.  15 

criminative  sensibility  required.  If,  for  example,  the  ratio 
for  one  subject  is  7 : 100  and  for  another  14 : 100,  the  first 
has  a  sensibility  to  pressure  differences  twice  as  acute  as 
the  second.  In  half  of  the  tests,  both  above  and  below,  the 
standard  weight  must  be  placed  upon  the  hand  first,  and  in 
half  the  weight  to  be  compared  with  it.  It  is  well  also  to 
distribute  the  determinations  of  the  differences  above  and 
below  so  that  they  shall  be  about  equally  affected  by 
practice  and  fatigue.  The  aim  should  always  be  to  keep  all 
the  conditions  of  the  experiment  as  constant  as  possible 
and  especially  to  have  them  the  same  for  the  weights  to  be 
compared.  Be  careful  in  putting  on  the  weights  that  the 
subject  does  not  recognize  a  difference  in  the  force  with 
which  they  strike;  also  that  suggestions  by  difference  of 
temperature  or  by  sounds  made  in  selecting  the  weights 
are  avoided. 

It  is  easy  to  see  that  this  method  has  some  disadvantages. 
First,  it  leaves  to  the  feeling  of  the  subject  what  the  just 
observable  difference  is,  and  this  feeling  is  liable  to  change 
from  subject  to  subject  and  in  the  same  subject  at  different 
times.  In  using  this  method  the  subject  must  know  the 
direction  of  the  change  that  he  is  to  recognize,  and  so  is 
somewhat  exposed  to  the  influence  of  expectant  attention. 
And  finally,  when  weights  are  found  that  are  just  observ- 
ably different,  it  is  possible  that  they  are  a  little  larger  than 
the  subject  could  just  recognize ;  that  is,  that  he  has  allowed 
himself  a  small  margin  for  security.  These  difficulties  may 
be  partially  obviated  by  a  more  rigorous  application  of  the 
method. 

Thus  in  making  the  tests  for  the  just  observable  differ- 
ences above  and  below,  weights  must  first  be  taken  that  are 
not  recognizably  different  from  the  standard,  and  must  then 
be  slowly  increased  or  decreased  till  just  observably  different. 
Subjective  equality  must  be  regarded  rather  than  objective 


16          LABORATORY   COURSE  IN  PSYCHOLOGY.          [26 

equality,  if  the  two  are  at  odds,  as  sometimes  happens.  To 
these  tests  two  others  must  be  added;  namely,  for  the  just 
wwobservable  differences  above  and  below,  the  operator  now 
selecting  a  weight  that  is  clearly  heavier  than  the  standard 
and  decreasing  it  gradually  till  it  can  just  no  longer  be 
recognized  as  different,  and  similarly  selecting  one  that  is  at 
first  clearly  lighter  than  the  standard  and  increasing  it  till 
it  seems  the  same.  The  average  of  the  four  tests,  just 
recognizably  different  and  just  ^recognizably  different,  is 
then  taken  for  the  ratio.  When  great  accuracy  is  required 
the  method  must  be  used  in  this  complete  form.  For  other 
methods  and  fuller  literature,  see  the  chapter  on  Weber's 
Law  below. 

Weber,  543-549;  Wundt,  3te  Aufl.,  I.,  343  ff.,  350;  4te  Aufl.,  I., 
336  f.,  341  ff. 

25.  Temperature  and  Pressure.     Cold  and  hot  bodies  feel 
heavier  than  bodies  of  equal  weight  at  a  normal  temperature. 

a.  For  cold,  take  two  dollar  pieces,  warm  one  until  it 
ceases  to  seem  cold ;  cool  the  other  to  10°  C.     Apply  alter- 
nately to  the  palm  of  the  hand,  letting  the  hand  rest,  mean- 
while, on  the  table  or  some  other  support  so  as  to  exclude 
"  muscle  sense."     The   cold  one  will  seem  much  heavier, 
perhaps  as  heavy  as  two  at  the  normal  temperature.     The 
same  experiment  may  be  tried  on  the  forehead  with  the  head 
supported. 

b.  For  heat  take  two  wooden  cylinders  of  equal  weight ; 
heat  one  to  a  high  temperature  by  standing  it  on  end  in  a 
metal  vessel  floating  in  a  water  bath.     Apply  the  cylinders 
on  end  alternately  to  the  back  of  the  hand  (supported)  be- 
tween the  metacarpal  bones  of  the  thumb  and  first  finger. 
The  hot  one  will  seem  heavier. 

Weber,  512,  551;  Szabadf oeldi ;  Funke,  320;  Dessoir,  304-306. 

26.  Pressure  Evenly  Distributed  over  a  Considerable  Area 
is  less  strongly  felt  than  pressure  upon  an  area  bordered  by 


30]  THE  DERMAL  SENSES.  17 

one  that  is  not  pressed.  Dip  the  hand  up  to  the  wrist  into 
water  (or,  better  still,  into  mercury)  of  normal  temperature, 
and  notice  that  the  sensation  of  pressure  is  strongest  in  a 
ring  about  the  wrist  at  the  surface  of  the  water ;  possibly 
stronger  on  the  volar  than  on  the  dorsal  side.  The  ring 
effect  is  unmistakable  when  the  hand  is  moved  up  and  down 
in  the  water. 

27.  Pressures  are  not  Equally  well  Perceived  in  all  Parts 
of  the  Body.     This  may  be  tested  with  weights  applied  some- 
what as  in  Ex.  24,  as  was  done  by  Weber,   but  a  simpler 
experiment  may  be  made  as  follows :  Find  the  pulse  at  the 
wrist ;  feel  it  with  the  finger  tips,  the  back  of  the  fingers, 
the  side  of  the  hand,  the  other  wrist,  the  lip,  and  the  tip  of 
the  tongue.     Try  the  pulse  in  the  temple  with  the  finger 
tips,  the  side  of  the  hand,  and  the  fore-arm.     Notice  that 
when  it  is  felt  by  another  person  the  experimenter  is  unable 
to  feel  it  subjectively. 

Goltz. 

28.  Kefinement  of  Active  Pressure  Sense.     Something  of 
the  refinement  of  the  pressure  sense  in  perceiving  the  un- 
evenness  of  surfaces  may  be  found  by  laying  a  hair  on  a 
plate  of  glass  or  other  hard,  smooth  surface  and  over  it  10 
or  15   sheets  of  writing-paper.     The  position  of  the   hair 
can  easily  be  felt  by  passing  the  finger  tips  back  and  forth 
over  the  paper. 

29.  The   Hairs   as    Organs  of   Touch.     The  finest  hairs 
respond  with   a  distinct   sensation   of   anticipatory  touch, 
when  they  are  moved,  and  probably  this  accounts  for  a  part 
at  least  of  the  differences  between  the  fore-arm  and  finger 
tips  found  in  Ex.  22.     Touch  a  few  single  hairs  and  observe 
the  sensation. 

Blaschko. 

30.  The  Feeling  of   Traction  or  Negative  Pressure  has 


18  LABORATORY  COURSE  IN  PSYCHOLOGY.          [31 

been  discriminated  by  some  authors,  but  has  rarely  been 
made  an  object  of  experiment.  It  is  to  be  observed  when 
viscid  substances  are  handled,  when  a  portion  of  the  skin  is 
brought  over  the  mouth  of  a  closed  vessel  and  the  air  ex- 
hausted, or  when  in  any  other  way  the  skin  is  lifted  from 
the  underlying  portions  of  a  member.  The  sensation  may 
be  studied  qualitatively  by  passing  a  thread  through  a 
small  bit  of  court-plaster,  knotting  it  on  the  gummed  side 
and  sticking  the  plaster  to  the  skin.  Traction  on  the  thread 
now  produces  the  sensation. 
Hall  and  Motora,  93  ff. ;  Bloch. 

GENERAL  SENSATIONS,  TICKLE,  AND  PAIN. 

These  topics,  though  clearly  of  very  great  psychological 
interest,  have  so  far  received  comparatively  little  careful 
study,  and  few  experiments  have  been  made  upon  them. 
They  are  not  exclusively  dermal  senses,  but  the  skin  offers 
the  most  convenient  field  for  the  study  of  the  two  to  be 
considered  here,  namely,  tickle  and  pain.  In  both  the 
experimenter  should  notice  the  subjective  cast  of  the  sensa- 
tions. Our  eyes  and  ears  give  us  information  about  colored 
and  sounding  things,  but  tickle  and  pain  let  us  know  that 
we  are  being  tickled  or  hurt  by  something. 

31.  Tickle.  Two  sorts  of  tickle  are  easily  distinguish- 
able, a  deep-seated  tickle  located  in  the  rib  region,  which 
seems  more  strongly  developed  in  children,  and  responds  to 
rather  strong  stimulation,  and  a  superficial  tickle  much 
more  widely  distributed,  and  responding  to  slight  stimuli 
only.  The  latter  sort  is  that  regarded  in  this  group  of 
experiments. 

a.  Touch  very  lightly  the  different  parts  of  the  face,  es- 
pecially about  the  eyes,  the  margin  of  the  lips  and  the 
opening  of  the  ears  with  the  tip  of  a  light  wisp  of  paper 
and  notice  the  tickle  sensations.  Notice  the  apparent 


32]  THE  DERMAL  SENSES.  19 

disproportion  between  the  stimulus  and  the  resulting  sen- 
sation, the  wide  and  indefinite  irradiation,  and  the  long 
after-image. 

b.  Touch  the  same  parts  as  lightly  as  possible  with  the 
tip  of  a  penholder  or  the  finger,  and  then  with  the  same 
instrument  while  exerting  at  the  same  time  a  moderate  pres- 
sure.    Notice  the  difference  in  effect ;  notice  also  that  the 
tendency  to  rub  a  tickled  surface  is  a  tendency  to  use  a 
greater  stimulus  to  remove  the  effects  of  the  less.     Notice 
also,  when  feeling   a  tendency  to  sneeze,  that  the  sneeze 
can  be  wholly  prevented  by  firm  pressure  or  rubbing  of  the 
sides  of  the  nose  or  the  adjacent  parts  of  the  face. 

c.  Tickle  is  apparently  a  summation  phenomenon.    Touch 
the  tip  of  the  tongue  lightly  with  the  prong  of  a  tuning- 
fork  at  rest  and  notice  the  after-image,  which,  however,  has 
no  tickle  in  it.     Then  strike  the  fork  and  touch  it  to  the  tip 
of  the  tongue.     Compare  the  effects. 

d.  The  ticklability  of  adjacent  parts  of  the  body  is  quite 
markedly  different.     Test  with  the  tuning-fork,  striking  it 
and  applying  it  gently  to  the  tip,  sides,  and  middle  of  the 
upper  surface  of  the  tongue  and  to  the  lower  surface. 

32.  Pain.  a.  Slow  conduction.  Remove  the  shoe  and 
strike  a  smart  blow  with  a  light  rod  on  the  sole  of  the  foot, 
or  on  a  corn ;  the  pain  will  be  perceived  noticeably  later  than 
the  first  sensation  of  contact,  separated  from  it  perhaps  by 
an  almost  empty  interval.  This  delay  is  probably  due  to 
the  same  cause  as  the  secondary  after-image  of  touch  in 
Ex.  11. 

b.  Temperature  pains.  A  given  increase  of  heat  above 
the  blood  temperature  is  more  effective  in  causing  pain  than 
an  equal  decrease.  Compare  the  effects  of  plunging  the 
hand  into  water  at  10°C.  and  at  60°.  Use  a  considerable 
quantity  of  water  and  do  not  allow  the  hand  to  remain  too 


20        LABORATORY  COURSE  IN  PSYCHOLOGY.        [32 

long  in  the  water,  for  its  sensibility  to  pain  as  well  as  to 
temperature  is  decreased  by  fatigue. 

Experiments  on  pain  can  likewise  be  made  with  electrical 
stimulation  and  pressure.  These  are  especially  suitable  for 
determining  the  relative  sensibility  of  different  subjects. 
The  first  can  easily  be  tried  with  the  sliding  induction  coil, 
by  applying  the  electrodes  to  the  surface  to  be  tested  and 
then  gradually  pushing  the  secondary  coil  towards  the 
primary  till  the  stimulation  becomes  painful.  For  appara- 
tus, see  the  chapter  on  apparatus  below. 

Weber,  569  if. ;  Dessoir,  Beaunis,  Lombroso,  Mantegazza,  Preyer,  89. 


BIBLIOGRAPHY. 

IN  the  following  bibliography,  and  those  appended  to  later  chapters, 
the  aim  has  not  been  to  make  an  exhaustive  list,  but  rather  to  give 
a  number  of  the  more  important  references  by  which  the  student 
may  begin  the  study  of  original  sources  if  he  desires.  For  the  same 
reason  the  list  has  not  been  kept  strictly  to  works  where  the  experi- 
ments of  the  chapter  are  discussed,  but  a  few  other  important  refer- 
ences have  been  given.  If  any  one  wishes  to  increase  the  list,  he 
can  easily  do  so  from  the  reviews  in  the  various  philosophical  and 
psychological  journals,  and  from  the  classified  bibliography  on  psy- 
chology and  the  physiology  of  the  sense  organs  published  yearly  in 
the  Zeitschrift  fur  Psychologic,  beginning  with  the  literature  of 
1889.  For  the  older  literature  the  rich  citations  of  Volkmann's 
Lehrbuch  der  Psychologic  may  be  consulted.  Much  psychological 
literature  appears  at  present  in  the  physiological  periodicals.  The 
Centralblatt  fur  Physiologic  contains  reviews  and  an  annual  bibli- 
ography of  general  physiology,  with  sections  on  the  physiology  of  the 
senses  and  physiological  psychology,  and  has  done  so  since  its  begin- 
ning, in  1887.  Hermann's  Handbuch  der  Physiologic  makes  many 
references  to  literature,  and  each  important  section  in  Beaunis' s 
J^lements  de  Physiologic  humaine  is  followed  by  a  bibliography. 
Hoffmann  and  Schwalbe's  Jahresberichte  tiber  die  Fortschritte  der 


UNIVERSITY 

OF        y 

*THE  DERMAL   SENSES.  21 

Anatomie  und  Physiologie  gives  bibliographies  and  summaries  of 
literature  since  1872.  The  Index  Medicus,  now  in  its  fifteenth 
volume,  has  listed  all  current  medical  literature  since  1879  ;  and  in 
connection  with  this  may  be  mentioned  the  mammoth  Index  Cata- 
logue of  the  Library  of  the  Surgeon-General's  Office,  United  States 
Army,  of  which  thirteen  large  volumes  have  so  far  been  published 
(September,  1893),  extending  from  A  to  Sutngin.  The  descriptive 
pamphlet  of  the  Harvard  Psychological  Laboratory  also  contains, 
in  a  ten-page  appendix,  a  classified  list  of  psychological  literature. 

ARISTOTLE  :  nepi  Ewwiuv,  c.  2,  Bekker,  460.  Also  in  German  trans- 
lation by  Johannes  Miiller,  as  an  appendix  to  his  Ueber  die 
phantastischen  Gesichtserscheinungen.  Coblenz,  1826. 

AUBERT  UND  KAMMLER:  Moleschotf s  Untersuchungen,  V.,  1859, 
145. 

BEAUNIS  :  Les  Sensations  internes,  Paris,  1889. 

BLASCHKO  :  Zur  Lehre  von  den  Druckempfindungen,  Verhandl.  d. 
Berliner  physiol.  Gesell.,  Sitz.  27  Marz,  1885.  Du  Bois-Bey- 
mond's  Archiv,  1885,  349. 

BLIX  :  Experimentelle  Beitrage  zur  Losung  der  Frage  tiber  die 
specifische  Energie  der  Hautnerven,  Zeitschrift  fur  Biologic, 
XX.,  1884,  141-156. 

BLOCH:  Recherches  experimentales  sur  les  sensations  de  traction 
et  de  pression  cutanees,  Archives  de  Physiologie,  Ser.  5,  III., 
1891,  322-333. 

BRONSON  :  The  Sensation  of  Itching,  Medical  Record,  XXX VIII., 
1890,  No.  1041,  Oct.  18,  425-429. 

DESSOIR:  Ueber  den  Hautsinn,  Du  Bios-Reymond\<*  Archiv,  1892, 
175-339.  See  review  of  this  paper  by  Goldscheider,  Zeitschrift 
fur  Psychologie,  V.,  1893,  117-122. 

DONALDSON:  On  the  Temperature-sense,  Mind,  X.,  1885,  399-416. 

Du  BOIS-REYMOND,  RENE  :  Ueber  chemische  Reizung  des  Tempera- 
tursinnes.     Verhandlungen  der    Berliner   physiol.    Gesellsch., 
Sitz.  11  Nov.  1892.     Du  Bois-Reymond's  Archiv.,  1893, 187-190. 
FECHNER:  Elemente  der  Psychophysik,  I.,  201-211  (temperature). 

FUNKE:  Der  Tastsinn  und  die  Gemeingefiihle,  Hermann's  Hand- 
buch  der  Physiol.,  Vol.  III.,  pt.  2,  289-414. 


22         LAEOEATOEY  COUESE  IN  PSYCHOLOGY. 

• 

GOLDSCHEIDEK  :  A.  Ueber  Warme-,  Kalte-  und  Druckpunkte, 
Yerhandl.  d.  Berliner  physiol.  Gesell.,  Sitz.  13  Marz,  1885. 
Du  Bois-Eeymond' 's  Archiv,  1885,  340-345. 

B.  Neue  Thatsachen   tiber  die    Hautsinnesnerven.   Ibid.   1885, 
Supplement  Band,  1-110,  5  plates. 

C.  Zur  Dualitat  des  Temperatursinns,  Pfluger's  Archiv,  XXXIX., 

1886,  96-120.     (On  Herzen's  experiments.) 

D.  Ueber  die  specifische  Wirkung  des  Menthols  auf  die  Tempera- 
tur-Nerven,    Verb.  d.  Berliner  physiol.  Gesell.,  9  April,  1886. 
Du  Bois-Eeymond' s  Archiv,  1886,  555. 

E.  Histologische  Untersuchungen  iiber  die    Endigungsweise  der 
Hautsinnesnerven  beim   Menschen.     Ibid.    1886,  Supplement- 
Band,  191-231. 

F.  Die  Einwirkung  der  Kohlensaure  auf  die  sensiblen  Nerven  der 
Haut,  Verb.   d.  Berliner  physiol.  Gesell.,  4  Nov.   1887.     Ibid. 

1887,  575-580. 

G.  Ueber  die  Topographic  des  Temperatursinns,  Yerh.  d.  Ber- 
liner physiol.  Gesell.,  Sitz.  1  Juli,  1887.     Ibid.  1887,  473-476. 

H.  Ueber  die  Summation  von  Hautreizen,  Verhandlungen  der  phy- 
siol. Gesellsch.  zu  Berlin,  Sitz.  31  Oct.  1890.  Ibid.  1891,  164- 
169. 

7.  Die  Lehre  von  den  specifischen  Energieen  der  Sinnesnerven, 
Berlin,  1881.  A  forty  page  dissertation,  containing  full  refer- 
ences to  literature. 

GOLTZ  :  Ein  neues  Yerf ahren  die  Scharfe  des  Drucksinns  der  Haut 
zu  priifen,  Centralblatt  fur  med.  Wiss.,  1863,  No.  18,  273-276. 

HALL  AND  DONALDSON:  Motor  Sensations  of  the  Skin,  Mind,  X., 
1885,  557-572. 

HALL  AND  MOTOR  A:  Dermal  Sensitiveness  to  Gradual  Pressure 
Changes,  American  Journal  of  Psychology,  L,  1887,  72-98. 

HEKING:  Der  Temperatursinn,  Hermann's  Handbuch  der  Physi- 
ologic, Yol.  III.,  pt.  2,  415-439. 

HEKZEN:  Ueber  die  Spaltung  der  Temperatursinnes  in  zwei  geson- 
derte  Sinne,  Pfluger's  Archiv,  XXXYIIL,  1886,  93-103. 

HOPPE  :  Das  Aristotelische  Rathsel  der  mit  den  gekreuzten  Finger- 
spitzen  gefiihlten  Kugel,  Wiener  med.  Presse,  1888,  Nos.  22,  23, 
785,  827. 


THE  DERMAL    SENSES.  23 

JAMES  :  Principles  of  Psychology,  New  York,  1890. 

LADD  :  Elements  of  Physiological  Psychology,  New  York,  188*7. 

LOEB:  Untersuchimgen  iiber  den  Fiihlraum  der'  Hand,   Pflugers 

Archiv,  XLL,  1887,  107-127. 
LOMBROSO:  Algometria  elettrica  nel  uomo  sano  e  alienato,  Milano, 

1867. 
(Lombroso  und  Ottolenghi)  Die  Sinne  der  Verbrecher,  Zeitschrift 

fur  Psychologic,  II.,  1891,  337  ff. 
LOTZE  :  A.  Medicinische  Psychologic,  Leipzig,  1852. 
B.   Outlines  of  Psychology,  translated  by  Herrick,  Minneapolis; 

also  by  Ladd,  Boston,  1885. 

MANTEGAZZA:  La  Physiologic  de  la  Douleur,  Paris,  1888. 
PREYER:  Ueber  den  Farben-  und  Temperatur-Sinn  mit  besonderer 

Riicksicht  auf  Farbenblindheit,  Pfluger's  Archiv,  XXV.,  1881, 

especially  pp.  75-92. 
QUINCKE  :  Ueber  Mitempfindungen  und  verwandte  Yorgange,  Zeitsch. 

fur  klin.  Medicin,  XYIL,  1890. 
SCHWANER  :  Die  Priifung  der  Hautsensibilitat  vermittelst  Stimm- 

gabeln  bei   Gesunden   und   Kranken,  Inaug.  Diss.,  Marburg, 

1890,   37.     Review  with    table  of    sensibility,  Zeitschrift  fur 

Psychologic,  II.,  1891,  398. 
SERGI:  Su  alcuni  caratteri  del  senso  tattile,  Bivista   di  Filosofia 

Scientiftca,  1891.     Same  paper  in  German,  Zeitschrift  fur  Psy- 
chologic, III.,  1892,  175-184. 
STUMPF:  Zum  Begriff  der  Lokalzeichen,  Zeitschrift  fur  Psychologic, 

IY.,  1892,  70-73. 

SZABADFOELDI:  MoleschotV 's  Untersuchungen,  IX.,  1865,  631. 
YIERORDT:  Physiologic  des  Menschen,  Tubingen,  1877,  340  ff. 

WEBER:  Der  Tastsinn  und  das  Gemeingefiihl,  Wagner's  Handwor- 

terbuch  der  Physiologic,  III.,  2,  481-588. 
YON  WITTICH:    Bemerkungen  zu  Preyers  Abhandlung    iiber  die 

Grenzen   des  Empfindungsvermogens    und  Willens,  Pfluger's 

Archiv,  II.,  1869,  329-350. 
WUNDT:    Grundzuge   der   physiologischen  Psychologic,   3te   Aufl., 

Leipzig,  1887,  I.,  391  ff.,  II.,  5   ff.;  4te   Aufl.,  Leipzig,  1893, 

L,  410  ff, 


24         LABORATORY  COURSE  IN  PSYCHOLOGY. 

For  further  bibliographical  references  see  especially  the  citations  of 
the  following  authors:  On  Touch,  Dessoir  and  Goldscheider. 
On  Temperature,  Dessoir,  Donaldson,  Goldscheider.  On  Pres- 
sure, see  bibliographies  following  the  chapter  on  Weber's  Law 
and  the  account  of  pressure  sense  apparatus  below.  On  Pain 
in  general,  see  Bain,  Mind,  Ser.  2,  I.,  1892,  161;  Marshall, 
Ibid.  Ser.  1,  XIV.,  1889,  511;  XYL,  1891,  327,  470;  Ser.  2,  L. 
1892,358,  453;  Philosophical  Review,  L,  1892,  625;  Nichols, 
Ibid.  L,  1892,  403,  518. 


KIN^STHETIC  AND   STATIC  SENSES.  25 


CHAPTER   II. 
Kinaesthetic  and  Static  Senses. 

THIS  group  of  senses  furnishes  us  data  for  the  perception 
of  the  positions  and  motions  of  our  members  and  of  the 
body  as  a  whole,  and  plays  a  leading  part  in  the  perception 
of  space.  It  includes  some  senses  whose  existence  or 
efficiency  is  disputed  (Innervation  Sense l  and  Muscle  Sense), 
and  others  whose  independence  has  only  of  late  been  gener- 
ally recognized  (Joint  Sense  and  Tendon  Sense).  All  are 
closely  united  with  one  another  and  with  pressure  and 
contact,  and  some  are  hardly  ever  dissociated  except  by 
disease.  This  chapter  is  necessarily  limited  to  the  experi- 
mental side  of  the  subject  and  to  the  simpler  experiments 
to  be  found  there.  Many  of  the  most  important  psycho- 

1  The  term  "  innervation  sense  "  must  not  be  taken  too  strictly  as  meaning  a 
wholly  independent  sense  of  motor  discharge,  as  it  has  often  been  taken.  Says 
Wundt,  in  his  last  edition  (4te  Aufl.  I. ,425):  "Manifold  observations  make  it 
probable  that  the  central  components  of  the  sensations  accompanying  active 
movements  have  their  origin  in  the  memory-images  of  movements  previously 
executed,  which  partly  initiate,  partly  accompany,  each  voluntary  movement. 
Since  memory-images  possess  qualitatively  the  same  sensory  content  as  the 
original  perceptions,  such  central  sensations  of  effort  and  movement  (Kraft-  und 
Bewegungsempfindungen)  will  under  normal  conditions  blend  completely  with  the 
more  intense  peripheral  sensations  of  the  same  kind;  they  will,  however,  produce 
an  independent  effect,  if  from  any  cause  the  peripheral  sensations  fall  away.  It 
would  be  proper,  therefore,  to  give  up  the  term  "  innervation  sensations  "  for  the 
sensations  in  question;  because  it  is  liable  to  convey  the  false  impression  that 
these  are  sensations  which  in  and  for  themselves,  without  any  relation  to  the 
peripheral  components  of  the  sensations  of  effort  and  movement,  accompany 
the  motor  innervation.  This  assumption,  which  as  a  rule  has  formerly  been  con- 
nected with  the  notion  of  "innervation  sensations,"  is,  however,  very  improb- 
able." Cf .  also  p.  431,  and  in  the  third  edition  I.,  p.  405  ff. 


26         LABORATORY  COURSE  IN  PSYCHOLOGY.        [34 

logical  problems  involve  the  motor  sensations  of  the  eye, 
some  of  which  are  considered  in  Chap.  VII. 

MUSCLE  SENSE,  Kraftsinn. 

Whether  there  are  any  specific  muscular  sensations 
distinct  from  those  that  come  from  other  parts  of  the 
member  in  motion  cannot  now  be  asserted  with  positive- 
ness  ;  but  even  if  there  be  such,  the  part  that  they  play  in 
our  ordinary  motor  perceptions  is  probably  a  minor  one. 
The  term  "muscle  sense,"  however,  has  been  used  to  desig- 
nate the  whole  group  of  motor  sensations,  and  is  here  re- 
tained for  that  purpose. 

33.  Lifted  Weights.      a.  Weights    lifted    slowly   seem 
heavier   than   the  same   weights  lifted   rapidly.     Lift   the 
same  weight  twice,  lifting  it  first  at  the  most  natural  and 
convenient  rate,  and  the  second  time  very  slowly,  beginning 
with   much   less   than   the  necessary  effort  and   gradually 
increasing  it  till  the  weight  rises. 

b.  Lift  a  moderate  weight  with  one  hand  and  at  the  same 
time   clench   the   other    sharply.     The   weight   will   seem 
lighter  than  when  110  simultaneous  effort  is  made. 

c.  Repeat  Ex.  23,  using  active  .lifting  instead  of  pressure 
in  comparing  the  weights. 

Charpentier;  on  a,  Goldscheider,  A,  186. 

34.  Discriminative  Sensibility  for  Lifted  Weights. 

a.  Find  by  the  method  of  experiment  used  in  Ex.  24  what 
is  the  just  observable  difference  above  and  below  a  standard 
weight  of  100  grams,  when  the  weights  are  lifted  instead  of 
merely  being  allowed  to  press  upon  the  skin.  In  this  ex- 
periment lift  the  weights  successively  with  the  same  hand. 
The  weights  must  be  placed  near  together  within  convenient 
reach,  and  care  must  be  taken  that  both  are  lifted  at  the 
same  rate  and  to  the  same  height.  Let  the  subject  lift  one 


35]  KIN^ESTHETIC  AND   STATIC  SENSES.  27 

weight  and  then  the  other,  and  render  his  decision  after 
once  lifting  each.  In  half  of  the  trials  let  the  standard 
weight  be  placed  at  the  left  side  of  the  weight  to  be  com- 
pared and  be  lifted  first ;  in  the  other  half  let  the  weight  to 
be  compared  stand  at  the  left  and  lead  in  the  lifting. 

b.  Kepeat  the  experiment,  letting  the  subject  lift  the 
standard  with  one  hand,  and  the  comparison  weight  with 
the  other,  keeping  the  same  hand  for  each  during  each  set 
of  trials  (that  is,  during  a  determination  of  the  just  ob- 
servable difference  above  and  below),  but  combining  a  num- 
ber of  sets  with  the  standard  in  the  right  hand  with  an 
equal  number  in  which  it  is  in  the  left.  Find  also  from  the 
figures  the  ratios  when  the  standard  is  in  the  right  hand 
and  when  it  is  in  the  left  hand,  for  use  in  Ex.  35.  Com- 
pare the  ratios  found  in  these  experiments  with  that  found 
in  Ex.  24. 

In  these  experiments  the  sense  of  pressure  might  be  ex- 
pected to  co-operate ;  but  when  it  is  excluded,  or  put  at  a 
relative  disadvantage,  the  sensibility  for  differences  of  lifted 
weights  is  not  diminished.  Weber's  method  of  excluding 
the  pressure  sense  was  to  wrap  the  weights  in  pieces  of 
cloth,  and  lift  them  by  the  four  corners  together.  The 
pressure  on  these  corners  can  be  changed  at  will,  irrespec- 
tive of  the  heaviness  of  the  weight  lifted. 

For  fuller  literature  on  lifted  weights,  see  the  chapter  on 
Weber's  Law  below. 

Weber,  546-547;  Miiller  und  Schumann;  James,  II.,  189  ff.,  486  ff. ; 
Beaunis;  Wundt;  Fullerton  and  Cattell. 

35.  Adjustment  of  the  Motor  Discharge.  After  having 
performed  the  second  part  of  Ex.  34,  compare  the  standard 
weight  with  a  very  much  heavier  weight,  e.g.,  2  kg.,  with 
all  the  circumstances  of  actual  careful  judgment.  Practise 
this  judgment  thirty  times,  leaving  a  longer  time  between 


28         LABOEATOEY  COUESE  IN  PSYCHOLOGY.        [36 

the  individual  comparisons  than  between  liftings  of  the 
weights  compared.  Then  at  once  return  to  the  smaller 
weights,  giving  the  standard  to  the  same  hand  as  before, 
and  to  the  hand  that  has  just  been  lifting  the  2  kg.  the 
weight  to  be  compared.  Not  only  will  the  weight  just  rec- 
ognizably heavier  before  seem  considerably  lighter  than  the 
standard,  but  also  still  heavier  weights  will  seem  so.  This 
time  the  tests  must  be  few,  not  more  than  three  or  four.  If 
more  tests  are  desired,  practise  the  comparison  of  the  stand- 
ard and  2  kg.  weight  again  ten  times  before  taking  them. 
By  the  practice  the  nervous  centres  discharging  into  the 
muscles  that  raise  the  2  kg.  weight  become  accustomed  to 
a  larger  discharge  than  that  required  for  the  small  weights 
and  do  not  at  once  re-adapt  themselves,  but  supply  too  great 
a  discharge.  The  weight  now  rises  with  greater  rapidity 
than  the  standard,  and  is  consequently  pronounced  lighter 
(Miiller  and  Schumann),  or  the  balance  between  the  ex- 
tensors and  flexors  that  was  suited  to  raising  the  heavier 
weight  is  not  suited  to  the  lighter  weight,  and  the  second  is 
pronounced  lighter  because  of  the  strain  in  the  extensors 
necessary  to  restore  the  balance  (Delabarre).  This  experi- 
ment seems  conclusive  against  a  well-developed  and  inde- 
pendent innervation  sense ;  for  if  there  were  any  sensation 
of  nervous  discharge,  we  ought  to  know  when  we  go  from 
a  very  heavy  to  a  light  weight  that  the  discharge  is  dis- 
proportionate ;  but  we  do  not. 

Miiller  und  Schumann;  but  cf.  also  Fullerton  and  Cattell,  131, 
and  Delabarre. 

INNERVATION  SENSE, 

36.  Simultaneous  Movements.  The  evidence  most  fre- 
quently offered  in  support  of  a  special  innervation  sense  is 
clinical  and  therefore  beyond  the  scope  of  this  course.  Ex- 
periments of  the  type  of  the  following  have  been  brought 
forward,  but  their  interpretation  has  been  disputed. 


36]  KIN  ESTHETIC  AND   STATIC  SENSES.  29 

a.  Stand  erect  before  the  blackboard,  with  the  eyes  closed 
and  coat  off,  if  it  interferes  with  free  motion  of  the  arms. 
Draw  with  each  hand,  using  both  at  once,  a  conventional  leaf 
pattern  like  those  in  the  annexed  cut,  drawing  always  from 
a  to  b.     In  drawing,  try  to  make  the 

lobes  of  the  leaf  of  equal  size,  like 
those  in  Fig.  1;  draw  each  with  a 
single  simultaneous  "  free-hand  "  mo- 
tion of  the  arms,  that  is,  draw  each 
with  a  single  volitional  impulse  di- 
rected equally  to  the  two  sides ;  the 
last  point  is  important.  First  draw 
a  pair  of  leaves,  beginning  them  with 
the  hands  before  the  shoulders  at  the 
same  height ;  the  result  will  be  ap-  \  Fig.  2. 

proximately  like  Fig.  1.      Next  draw 
a  pair  with  one  hand  about  a  foot  higher  than  before,  the 
other  about  a  foot  lower  ;  the  result  will  be  like  Fig.  2. 

b.  Bring  the  hands  again  to  the  position  used  in  drawing 
Fig.  1,  and  draw  a  pair  of  leaves  having  their  apices  right 
and  left.     The   leaves  will   be   symmetrical.     Next  begin 
with  one  hand  about  a  foot  farther  away  from  the  median 
plane  than  before  and  the  other  at  it,  but  both  at  the  same 
level.     Draw  as  before  ;  asymmetrical  leaves  will  be  the  re- 
sult.    Repeat  the  drawing  a  number  of  times,  sometimes  rais- 
ing or  extending  one  arm,  sometimes  the  other.     In  general 
it  will  be  found  that,  notwithstanding  the  intention  to  make 
equal  movements  of  the  hands,  the  motions  of  further  ex- 
tension in  the  extended  arm  and  of  further  flexion  in  the 
flexed  arm  are  too  short,  and  those  in  the  contrary  direction 
in  each  case  too  long.     The  argument  founded  on  this  ex- 
periment runs  as  follows  :  We  think  that  our  hands  execute 
equal  movements,  when  they  do  not,  because  we  are  con- 
scious of  willing  equal  movements,  and  unconscious,  or  only 


30         LABORATORY  COURSE  IN  PSYCHOLOGY.        [38 

inexactly  conscious,  of  those  actually  made.  If  we  per- 
ceived motion  of  our  members  by  the  skin,  joint,  and  muscle 
sensations  that  accompany  their  motion  (as  the  opponents 
of  the  innervation  sense  believe)  we  ought  to  know  the  ex- 
tent to  which  our  hands  are  moved  each  time,  and  not  to 
fall  into  the  illusion  that  we  find  in  these  experiments, 
Cf.  Ex.  44  d. 
Loeb,  B,  15  ff. 

37.  Illusory  Movement  in  an  Immovable  Member.     Lay 
the  hand  palm  downward  on  the  edge  of  the  table  or  on  a 
thick  book  so  that  the  last  three  fingers  shall  be  supported 
and  held  extended  while  the  thumb  and  first  finger  remain 
free.     Bend  the  first  finger  considerably  at  both  the  inner 
joints,  and  hold  it  in  position  with  the  other  hand.     The 
finger-tip  is  still  movable,  as  will  be  found  011  touching  it ; 
but  it  is  anatomically  impossible  to  move  it  voluntarily. 
When,  however,  the  effort  is  made  to  move  it  (the  eyes  be- 
ing closed),  there  is  a  sensation  of  motion,  though  no  actual 
motion  is  possible.     From  this,  an  inner  sense  of  motion 
(innervation   sense)    has   been   inferred.     When   operating 
upon  another  subject,  the  operator  may  hold  the  finger  in 
position,  and  require  the  subject  to  execute  with  the  corre- 
sponding finger  of  his   free  hand  a  motion  equal  to  that 
which  he  thinks  he  makes  with  the  one  that  is  held.     Ob- 
serve, however,  that  the  tendons  in  the  wrist  move,  and 
that  there  are  slight  movements  elsewhere  in  the  hand. 

Sternberg;  James,  II.,  105,  515,  footnote;  Goldscheider,  At  317. 

38.  Terrier's  Experiment.     That  the  feeling  of  effort  is 
largely,  if  not  entirely,  of  peripheral  rather  than  central  ori- 
gin, appears  from  such  experiments  as  the  following.    Hold 
the  finger  as  if  to  pull  the  trigger  of  a  pistol.     Think  vigor- 
ously of  bending  the  finger,  but  do  not  bend  it  j  an  unmis- 


39]  KIN^STHETIC  AND   STATIC   SENSES.  31 

takable  feeling  of  effort  results.  Repeat  the  experiment, 
and  notice  that  the  breath  is  involuntarily  held,  and  that 
there  are  tensions  in  other  muscles  than  those  that  would 
move  the  linger.  Repeat  the  experiment  again,  taking  care 
to  keep  the  breathing  regular  and  the  other  muscles  passive. 
Little  or  no  feeling  of  effort  will  now  accompany  the  imagi- 
nary bending  of  the  finger. 

Terrier,  382  ff.  (English  Ed.). 

On  Innervation  Sense  in  general,  besides  the  authors  already  men- 
tioned, see  :  Wundt,  3te  Aufl.  I.,  397  ff.,  4te  Aufl.  I.,  423  ff.;  James, 
II.,  486  ff.;  Goldscheider,  A,  206  ff. 

SENSATIONS  OF  MOTION,  JOINT  SENSATIONS. 

39.  Passive  Motion  at  the  Elbow.  Let  the  subject  rest 
his  fore-arm  flat  upon  the  arm-board  of  the  instrument 
(bringing  his  elbow 
over  the  hinge),  and 
close  his  eyes.  Let 
the  operator  then  raise 
or  lower  the  free  end 
of  the  arm-board 
slowly  by  pressing 
down  or  lifting  the 
counter  weight,  and 
require  the  subject  to 
announce  when  he  first 
perceives  the  motion 
of  his  fore-arm.  Re- 
cord the  angular  move- 
ment required  to  produce  a  just  observable  sensation. 
Notice  that  the  movement  seems  to  be  located  chiefly  in  the 
hand.  It  is  extremely  important  not  to  mistake  the  sensa- 
tion of  increased  pressure  or  of  jar  for  that  of  motion.  The 
rate  of  movement  will  be  found  important,  and  should  be 


32          LABORATORY  COURSE  IN  PSYCHOLOGY.        [41 

kept  as  constant  as  possible.     The  results  found  in  this 
way  are  rough ;  for  more  exact  methods  see  Goldscheider,  A. 

40.  Active  Movement  of  the  Last  Joint  of  the  Finger. 
The  joint  sensations  of  the  ringers  are  less  fine  than  those 
of  the  elbow,  but  are  more  convenient  for  demonstration  of 
active  flexion.     Fasten  a  piece  of  straw,  with  court-plaster 
or  otherwise,  to  the  finger-nail  of  the  middle  finger,  and  cut 
it  off  at  such  a  length  that  the  distance  from  the  joint  of 
the  finger  to  the  end  of  the  straw  shall  be  115  mm.     With 
that  radius  2  mm.  corresponds  to  about  1°  of  angular  meas- 
ure.    Eest  the  hand  on  a  thick  book,  letting  the  last  joint  of 
the  finger  extend  beyond  the  edge.     Set  up  a  millimeter 
scale  at  right  angles  with  the  straw.     Close  the  eyes  and 
make  the  least  possible  flexion  of  the  finger  at  the  last  joint, 
having  an  assistant  note  its  extent  on  the  scale.     Close  at- 
tention may  perhaps  be  able  in  both  the  active  and  passive 
movements  to  locate  the  sensation  in  the  joint,  but  more 
rigorous   experiments  are  required  to  show  its  character 
clearly,  and  to  prove  its  location. 

Goldscheider,  A. 

41.  Location  of  Movements,     a.  Motions  on  the  skin  can 
be  interpreted  either  as  the  movement  of  an  object  over  the 
surface  of  the  skin,  or  of  the  skin  over  the  surface  of  the 
object.     This  opens  the  way  for  illusions.     Have  an  assis- 
tant draw  a  pencil-point  gently  across  the  wrist  or  the  finger- 
tips of  the  observer,  who  sits  with  closed  eyes.     A  tendency 
to  interpret  the  sensation  as  motion  of  the  wrist  or  finger 
will  be  observed.     The  hand  and  arm  must  be  held  free,  so 
that  the  illusion  may  not  be  corrected  by  the  presence  of 
other  touch  sensations. 

b.  With  the  eyes  closed,  move  the  wrist  or  finger  over 
a  stationary  pencil-point.  In  this  case  the  point  also  seems 
to  be  in  motion  in  a  direction  contrary  to  that  of  the  hand. 


43]  KIN^STHETIC  AND   STATIC  SENSES.  33 

c.  When  the  movement  may  be  interpreted  as  belonging 
to  either  of  two  members,  it  may  be  credited  to  the  more 
mobile  of  the  two,  or  may  be  shared  by  both.     Rest  the 
finger  lightly  on  the  forehead ;  then,  taking  pains  to  keep  its 
position  fixed,  move  the  head  from  side  to  side.     There  is  a 
strong  tendency  to  credit  the  motion  to  the  finger  and  arm. 
Hold  the  last  three  fingers  close  together,  and  move  the  first 
away  from  them  and  toward  them  again.     All  will  seem  to 
move,  but  the  last  three  in  an  opposite  direction  to  the 
first. 

d.  Ex.  4  above  is  an  experiment  on  the  location  of  move- 
ments as  well  as  of  touches.     If  the  cane  is  swung  so  as  to 
describe  the  surface  of  a  cone  we  are  conscious  of  the  path 
described  by  its  point,  as  well  as  that  of  the  hand  holding  it. 

Cf.  Ex.  39  where  the  motion  of  the  whole  fore-arm  and 
hand  is  credited  chiefly  to  the  latter. 
Yierordt;  on  c,  Goldscheider,  J.,  181  ff. 

42.  Interrupted  Extent  may  seem  smaller  to  a  moving  mem- 
ber than  uninterrupted.     In  a  piece  of  cardboard  make  three 
pin-holes  in  a  line  separated  by  spaces  of  an  inch  and  a  half. 
Fill  one  of  the  spaces  with  pin-holes  a  quarter  of  an  inch  apart. 
Turn  the  card  over,  close  the  eyes,  and  move  the  finger-tip 
across  the  little  eminences  made  by  the   pin-holes.     The 
illusion  seems  more  marked  when  the  finger  moves  over  the 
interrupted  half  of  the  line  first.     Examine  the  card  visu- 
ally, and  notice  that  the  visual  illusion  is  in  the  directly 
opposite  sense.     As  in  the  similar  touch  experiment  above 
(Ex.  8)  the  results  are  apparently  not  equally  clear  for  all 
observers. 

James,  II.,  250. 

SENSATIONS  OF  RESISTANCE. 

43.  Illusory  Resistance,     a.  Hold  a  heavy  weight  by  a 
string  so  that  it  hangs,  with  the  arm  extended,  a  few  inches 


34          LABORATORY  COURSE  IN  PSYCHOLOGY.        [44 

above  the  floor,  or  better,  have  the  string  placed  in  the 
hand  by  an  assistant  so  that  the  length  of  the  string  may 
not  be  known  beforehand.  Lower  the  weight  rather  rapidly 
till  it  rests  on  the  floor  or  other  support.  As  it  strikes,  a 
sensation  of  arrest  will  be  perceived,  somewhat  as  though 
the  hand  were  suddenly  supported  by  a  light  rod.  The  illu- 
sion is  even  more  marked  when  the  string,  instead  of  being 
held  in  the  hand,  is  fastened  to  a  small  rod,  and  that  is 
held.  The  disturbing  noise  of  the  weight  may  be  obviated 
by  having  it  come  to  rest  on  a  cushion  or  in  a  box  of  sand. 
The  illusion  is  due  to  the  unexpected  strain  put  upon  the 
muscles  that  lower  the  arm  by  the  tension  of  those  that  have 
been  holding  the  weight.  This  feeling  of  arrest  is  prob- 
ably a  joint  sensation.  To  distinguish  this  sensation  from 
the  motion  sensations  of  the  joints,  Goldscheider  has  called 
it  a  "joint-pressure  sensation." 

b.  When  the  movement  of  the  rod  is  continued  downward 
beyond  the  point  at  which  the  sensation  of  arrest  is  felt,  a 
certain  difficulty  of   movement  may  still   be  observed,  as 
though  the  rod  were  moving  through  a  resisting  medium. 
This  sensation  Goldscheider  distinguishes  from  the  sensa- 
tion observed  in  a,  believing  it  to  be  the  true  sensation  of 
difficult  motion  (of  weight  and  heaviness  also)  and  credit- 
ing it  to  the  tendons. 

c.  Notice  something  similar  to  b  in  pouring  a  quantity 
of  mercury  rapidly  from  one  vessel  to  another. 

It  is  evident  that  such  illusions  as  these  speak  against 
the  existence  of  an  innervation  sense  in  the  common  accep- 
tation of  the  term. 

Goldscheider,  A,  164  ff.,  172  ff.,  D;  on  6,  A,  188;  Mach,  A,  70  if. 

BILATERAL  ASYMMETRIES  OF  POSITION  AND  MOTION. 

44.  Apparently  Symmetrical  Motions  of  the  arms.  In  all 
the  tests  of  this  group,  the  subject  should  be  kept  in  ignor- 


44]  KIN^STHETIC  AND   STATIC   SEN  HE  S.  35 

ance  of  the  nature  and  amount  of  his  errors  till  the  tests 
are  finished. 

a.  Hold  an  ordinary  cork  between  the  thumb  and  first 
two  fingers  of  each  hand.     Close  the  eyes  and  bring  the  two 
corks  together  at  arm's  length  in  the  median  plane  before 
the  face,  having  an  assistant  note  the  approximate  amount 
and  direction  of  the  error.     The  corks  should  be  brought 
together  rather  gently,  so  as  not  to  betray  the  character  of 
the  error  to  the  operator,  but  the  motions  of  the  arms  by 
which  they  are  brought  up  nearly  to  contact  should  be  free 
and  sweeping.     The  error  will  probably  be  found  rather  con- 
stant in  direction  uatil  the  operator  learns    to  correct  it. 
Try  bringing  the  corks  together  above  the  head,  and  also  in 
asymmetrical  positions. 

b.  Let  the  subject  seat  himself  at  a  table  with  the  milli- 
meter scale  before  him.    Set  a  pin  in  the  middle  of  the  scale, 
and  bring  the  pin  into  the  median  plane  of  the  subject  and 
make  the  scale  parallel  to  his  frontal  plane.     Let  the  sub- 
ject place  his  forefingers  on  either  side  of  the  pin,  and,  with 
closed  eyes,  try  to  measure  off  equal  distances  by  moving 
both  simultaneously  outward  along   the    scale.     Note   the 
result  in  millimeters ;  for  this  it  may  be  convenient  to  mark 
the  middle  point  of  the  finger-nails  with  an  ink-line.     A 
constant  excess  in  the  motion  of  one  hand  or  the  other  will 
often  be  found.     It  is  important  that  the  subject  should  not 
open  his  eyes  till  his  fingers  are  removed  from  the  scale ; 
for  he  will  find  it  difficult  not  to  correct  his  error  if  he 
knows  its  nature.     The   finger-tips  should  rest  lightly  on 
the  scale,  and  the  motions  should  be  made  from  the  shoulder 
by  a  single  impulse ;   if  they  are  too  slow,  and  the  subject 
attends  to  his  sensations  of  position,  the  errors  will  be  small 
and  uncertain.    The  left  hand,  it  is  said,  generally  makes  the 
greater  excursion  in  right-handed  persons  not  mechanics. 

c.  Repeat  the  tests,  having  the  motions  of  the  hands  made 


36         LABOEATOEY  COURSE  IN  PSYCHOLOGY.        [45 

successively  instead  of  simultaneously.  The  constant  differ- 
ence between  the  hands  will  probably  not  appear. 

d.  Let  the  subject  start  with  his  right  and  left  hand  each 
20  cm.  toward  its  own  side  of  the  median  plane,  and  try 
to  measure  off  equal  distances  on  either  side  of  those 
points,  moving  both  hands  at  once  in  the  same  direction. 
Distances  inward  will  be  made  too  large,  distances  outward 
too  small.  In  all  these  experiments  with  closed  eyes  we 
seem  inclined  to  judge  distance  rather  from  the  intention 
of  equal  motion  and  the  continuance  of  motor  sensations 
for  equal  times,  than  from  the  actual  peripheral  sensations. 

The  judgments  of  symmetry  of  position  and  motion  rest 
upon  very  complex  combinations  of  the  dermal  and  kin- 
aesthetic  sensations,  already  made  the  subject  of  experiment 
above.  As  a  result  of  this  complexity  the  experiments  of 
this  group  will  be  found  to  give  rather  variable  results, 
from  one  subject  to  another,  and  in  the  same  subject  at 
different  times. 

Hall  and  Hartwell ;  Loeb ;  Delabarre ;  Bloch. 

RECOGNITION  OP  THE  POSITION  OF  THE  BODY  AS  A  WHOLE. 
45.  Recognition  of  Direction.  In  this  experiment  it  is 
especially  desirable  that  the  subject  should  know  as  little  as 
possible  of  the  purpose  of  the  experiment.  Cause  him  to 
stand  erect  with  his  back  against  a  wall.  Choose  a  point 
on  the  opposite  wall  about  the  height  of  his  shoulders.  Let 
him  look  at  it,  and  then  require  him,  having  closed  his  eyes, 
to  point  to  it  as  exactly  as  possible  with  a  light  rod  held 
symmetrically  in  both  hands.  Cause  him  also  to  hold  the 
rod  vertically  arid  horizontally  in  the  median  plane ;  also 
horizontally  parallel  to  the  frontal  plane.  All  these  he  will 
probably  be  able  to  do  with  much  accuracy ;  or  if,  as  some- 
times happens,  he  shows  a  "personal  equation,"  his  error 
will  be  constant. 


45]  KIN^ESTHETIC  AND   STATIC  SENSES.  37 

a.  Cause  the  subject  to  repeat  the  experiment,  this  time 
turning  his  head  as  far  as  possible  to  the  left  after  closing 
his  eyes,  taking  pains  to  keep  his  shoulders  square.     Eepeat, 
causing  the  subject  to  turn  to  the  right.     In  both  cases  an 
error  will  be  observed,  the  subject  pointing  too  far  in  a 
direction  opposite  to  that  of  the  turning  of  the  head.     The 
subject  will  be  able  to  hold  the  rod  vertically,  or  horizontally, 
without  error.     Cause  the  subject  to  hold  the  rod  in  what 
he  thinks  is  a  horizontal  position,  in  the  median  plane  when 
his  head  is  thrown  well  back ;  when  bowed  well  forward. 
Illusions  like  those  observed  above,  affecting  directions  in 
the  plane  of  movement  of  the  head,  will  result.     Cause  the 
subject  to  hold  the  rod  in  what  he  thinks  is  a  horizontal 
position,  parallel  to  the  frontal   plane,  when  his  head  is 
bowed  to  the  right ;  when  bowed  to  the  left.     Illusions  sim- 
ilar to  those  in  the  previous  experiments  will  appear.     In  all 
these  cases  judgment  of  one  cardinal  direction  in  space  alone 
is  affected  ;  the  other  two  show  little  or  110  errors. 

b.  Repeat  the  first  part  of  experiment  ay  but  instead  of 
having  the  subject  point  to  the  designated  object,  have  him 
walk  toward  it,  keeping  his  shoulders  square,  his  eyes  shut, 
and  his  head  turned  to  one  side.     He  will  walk  more  and 
more  too  far  toward  the  side  away  from  which  his  head  is 
turned. 

c.  The  illusion  is  due,  at  least  in  the  case  of  turning  the 
head  about  a  vertical  axis,  to  the  position  of  the  eyes ;  the 
eyes  turn  farther  than  the  head  in  the  direction  in  which 
it  is  turned,  as  may  easily  be  observed  upon  any  other  per- 
son.    From  the  eyes  we  judge  the  position  of  the  head,  and 
thus  over  judging  it,  point  too  far  in  a  contrary  direction  in 
trying  to  point  to  the  required  object  (Delage).     The  illu- 
sions can  be  produced  by  motion  of  the  eyes  alone.     Holding 
the  head  erect,  and  taking  pains  not  to  move  it  when  moving 
the  eyes,  turn  the  closed  eyes  as  far  as  possible  to  the  right 


38          LABOEATOEY  COUESE  IN  PSYCHOLOGY.         [45 

or  left,  and  then  try  to  point  to  some  determined  object.  An 
error  like  that  in  a  will  be  observed.  Turning  of  the  eyes 
upward  or  downward  has  a  doubtful  result.  Instead  of 
closing  the  eyes,  they  may  be  kept  open  if  an  opaque 
screen  is  held  close  before  the  face.  Eepeat  a,  voluntarily 
turning,  the  eyes  as  far  as  possible  in  the  direction  opposite 
to  that  of  the  turning  of  the  head.  The  original  error  will 
probably  disappear,  or  be  found  to  have  changed  its  sign. 

For  this  illusion  another  eye  explanation  is  suggested  by 
Breuer,  namely,  that  in  such  extreme  turnings  of  the  eyes, 
their  actual  position  does  not  correspond  with  the  intended 
position,  but  comes  short  of  it.  We  infer  the  direction, 
however,  from  the  intended  position,  and  thus  fall  into  the 
error  in  pointing.  For  the  illusion  in  other  positions  of  the 
head  and  even  for  this,  his  own  preferred  explanation  is 
again  different,  and  is,  partly  based  on  the  following  experi- 
ment. 

d.  Close  the  eyes,  and  touch  the  tip  of  the  nose  or  the 
forehead  with  a  pin  or  a  pencil  while  the  head  is  in  the 
usual  position,  and  after  a  little  try  to  touch  the  same  spot 
again.  The  error,  if  any,  will  be  very  small.  Repeat  the 
touch  in  the  normal  position,  and  then  turn  the  head  to  the 
right  or  left  or  incline  it  toward  the  shoulder  or  forward  or 
backward.  After  holding  it  in  the  chosen  position  for  half 
a  minute,  attempt  to  touch  the  spot  again.  Gross  errors 
will  result  till  corrected  by  practice.  The  error  is  one  of 
underestimation,  and  should  by  itself  alone  produce  a  result 
directly  the  reverse  of  that  found  by  Delage.  Breuer,  how- 
ever, introduces  another  factor.  His  explanation  for  the 
inclined  positions  of  the  head  is  somewhat  as  follows  :  by 
means  of  the  otolith-apparatus  of  the  ear,  we  get  a  true 
perception  of  the  amount  of  inclination  of  the  head,  at  the 
same  time  that  we  get  the  erroneous  perception  just  men- 
tioned. The  only  way  in  which  we  can  harmonize  the 


46]  KIN  AESTHETIC  AND   STATIC   SENSES.  39 

conflicting  perceptions  is  by  altering  our  judgment  of  the 
vertical,  and  with  that,  of  course,  of  the  horizontal.  For  the 
movements  of  rotation  about  a  vertical  axis  the  semi-circular 
canals  (See  Exs.  47-49)  would  furnish  the  knowledge  of 
the  true  amount  of  turning,  and  from  a  similar  combination 
of  the  true  and  false  the  illusions  in  that  case  would  result. 

This  group  of  experiments,  except  perhaps  the  last,  when 
tried  under  the  ordinary  conditions  of  the  practice  labora- 
tory, seems  liable  to  considerable  individual  variation ;  but 
sufficient  care,  especially  as  to  the  position  of  the  eyes  in 
turning  to  the  right  and  left,  should  lead  to  a  tolerable 
degree  of  success. 

Aubert  (Delage),  17  ff.;  Loeb,  B,  20  f.,  31  f.;  Breuer,  270  ff. 

46.  Vertical  and  Horizontal  Positions  of  the  Body.  Secure 
the  subject  properly  upon  the  tilt-board,  and  have  him  close 
his  eyes.  Start  with 
the  board  vertical  (head 
up).  Require  the  sub- 
ject to  describe  his  po- 
sition. He  will  prob- 
ably announce  that  he 
is  then  leaning  forward 
slightly.  As  a  matter 
of  fact  he  is,  if  his  heels 
are  against  the  board. 
Turn  him  slowly  back- 
ward, and  require  him 
to  say  when  he  seems  to 
himself  vertical  (head 
up),  when  he  seems 

tilted  backward  at  an  angle  of  45°  -from  the  vertical,  when  at 
an  angle  of  (50°,  when  at  90°,  when  at  180°.  Two  classes  of 
illusions  will  be  found :  angles  of  less  than  40°  will  prob- 
ably seem  too  small ;  those  from  40°  to  60°  will  be  rightly 


40          LABOBATOBY  COURSE  IN  PSYCHOLOGY.        [49 

judged ;  those  beyond  60°  will  seem  too  large.  The  subject 
will  say  that  he  is  vertical,  head  downward,  when  he  is  yet 
30-60°  from  it.  The  subject  may  be  allowed  a  pillow  if  he 
desires  it. 

The  illusions  depend  in  large  measure  on  the  distribution 
of  pressure  on  the  soles  and  other  surfaces  of  the  body  and 
the  direction  of  pressure  of  the  movable  viscera  and  the 
blood. 

Aubert  (Delage),  40  ff. ;  Breuer,  270  f. 

SENSATIONS  OF  ROTATION. 

47.  Perception  of  Uniform  Rotations.     Let  the  subject 
be  seated  upon  the  rotation  table  with  closed  eyes,  blind- 
folded if  necessary.     Turn  the  table  slowly  and  evenly  in 
one  direction  or  the  other.     The  subject  will  immediately 
recognize  the  direction  and  approximately  the  amount  of 
rotation  when  the  rate  is  as  slow  as  2°  per  second,  or  even 
slower.      After  continued   rotation  at  a  regular   rate  the 
sensation  becomes  much  less  exact  or  entirely  fails.     This 
fact  has  been  generalized  by  Mach  in  the  law  that  only 
change  of  rate,  not  continuous  rotation,  is  perceived.     After 
some  pauses  and  short  movements  in  one  direction  and  the 
other,  the  subject  may  become  quite  lost,  and  give  a  totally 
wrong  judgment  of  the  direction  of  motion,  if  it  is  slow. 

48.  Illusion  of  Backward  Rotation.     Let  the  subject  be 
seated  as  before.     Rotate  him  a  little  more  rapidly  for  half 
a  turn,  and  then  stop  him  suddenly.     A  distinct  sensation 
of  rotation  in  the  opposite  direction  will  result.     Repeat, 
and  when  the  illusory  rotation  begins,  open  the  eyes.     It 
immediately  ceases.     Close  the  eyes  again,  and,  if  strong,  it 
again  returns. 

49.  Location  of  the  Organs  for  the  Perception  of  Rotation. 
a.  Repeat  the  first  part  of  Ex.  48,  letting  the  subject 


49]  KIN^STUETIC  AND   STATIC  SENSES.  41 

give  the  word  for  stopping.  At  the  same  instant  let  him 
incline  his  head  suddenly  backward  or  forward,  or  lay  it 
upon  one  shoulder  or  the  other.  The  axis  of  rotation  of 
the  body  will  appear  to  change  in  a  direction  opposite  to 
that  of  the  inclination  of  the  head ;  i.e.,  if  the  head  is  in- 
clined to  the  right,  the  axis  seems  to  incline  to  the  left. 
The  feeling  is  as  if  the  body  were  rotating  in  the  surface 
of  a  cone  in  a  direction  contrary  to  that  of  the  first  rotation. 
The  head  dictates  the  apparent  axis  of  rotation.  The  same 
illusion  occurs  if  the  head  is  inclined  during  the  actual 
rotation  and  straightened  at  the  word  for  stopping.  Turning 
the  head  to  the  right  or  left  introduces  no  such  illusion, 
because  it  does  not  change  the  axis  of  rotation  of  the  head. 
The  illusion  conies  out  with  very  disagreeable  strength 
when  the  rotation  is  rapid,  and  the  subject  changes  the 
position  of  his  head  during  the  rotation. 

b.  Let  the  subject  lie  upon  his  side,  and  rotate  him  rather 
rapidly  till  the  sensation  of  rotation  becomes  faint  or  disap- 
pears. Then  let  him  turn  suddenly  upon  his  back  or  upon 
his  other  side.  Turning  upon  his  back  starts  rotation  about 
a  new  axis,  and  it  is  felt  in  its  true  sense,  while  the  rotation 
about  the  previous  axis  is  felt  as  an  illusion  in  its  reverse 
sense.  The  resulting  perception  combines  both.  Turning 
completely  over  reverses  the  direction  of  motion  completely, 
and  the  combined  sensation  and  illusion  produce  a  corre- 
spondingly powerful  effect. 

The  change  of  the  apparent  axis  of  rotation  with  the 
change  of  position  of  the  head  points  to  the  location  in  the 
head  of  the  organ  for  such  sensations.  For  the  experiments 
by  which  the  semicircular  canals  are  indicated  as  this  organ, 
and  the  arguments  pro  and  con,  see  the  literature  cited  by 
Aubert,  Ayres,  and  others. 

On  the  last  three  experiments,  see:  Aubert  (Delage),  49  ff. ; 
Brown;  Mach;  Wundt,  3te  AufL,  I.,  211  f.;  II.,  24,139. 


42         LABORATORY  COUESE  IN  PSYCHOLOGY.        [50 

50.  Another  Illusion  of  Kotation  (Purkinje's  dizziness) 
is  due  to  involuntary  motions  of  the  eyes.  Let  the  subject 
whirl  rapidly  on  his  heels  with  his  eyes  open  till  he  begins 
to  be  dizzy.  At  first  objects  about  him  seem  at  rest,  then 
to  be  turning  in  the  opposite  direction.  Let  him  now  stop 
and  look  at  an  even  surfaced  wall  while  the  experimenter 
carefully  observes  his  eyes,  picking  out  some  clearly  marked 
fleck  or  spot  as  a  point  of  observation.  To  the  subject  the 
surrounding  objects  will  seem  to  continue  to  move  in  the 
same  direction  as  before  ;  i.e.,  in  a  direction  contrary  to  his 
previous  rotation;  the  experimenter  will  see  the  subject's 
eyes  executing  slow  motions  in  one  direction  (in  the  direc- 
tion of  the  original  motion  of  the  subject)  alternating  with 
rapid  motions  in  the  other.  The  subject  himself  may  be 
able  to  perceive  a  corresponding  irregularity  of  motion  in 
the  spots  upon  the  wall  at  which  he  looks.  He  can  easily 
observe  the  motions  of  his  own  eyes  if  he  looks  fixedly  for 
twenty  or  thirty  seconds  at  a  flame  or  a  strip  of  white  paper 
in  a  bright  light  before  beginning  his  rotation ;  the  after- 
image (see  Chapter  V.)  thus  produced  remains  fixed  on  the 
retina,  and  its  apparent  movements  betray  the  motions  of 
the  eye.  If  the  eyes  are  closed  after  the  rotation,  the  image 
will  seem  to  move  in  one  direction,  and  rather  slowly.  The 
illusion  rests  upon  the  subject's  unconsciousness  of  the 
slow  motions  of  his  eyes.  It  is  probable  that  these  eye 
motions  and  the  sensations  of  attempted  restoration  of  equi- 
librium in  other  parts  of  the  body  are  reflexly  caused  by  the 
disturbance  in  the  semicircular  canals. 

It  should  be  noticed  that  this  illusion  is  the  exact 
reverse  of  that  found  with  closed  eyes  in  Ex.  48.  There  the 
subject  feels  a  rotation  of  his  own  body  contrary  to  that  it 
previously  received.  If  he  was  turned  at  first  in  the  direction 
of  the  hands  of  a  watch,  on  being  stopped  he  would  seem  to 
be  turning  in  a  direction  contrary  to  the  hands.  If  these 


51]  KIN^STHETIC  AND    STATIC   SENSES.  43 

motions  were  transferred  to  objects  about  him,  they  would, 
during  the  rotation,  seem  to  move  contrary  to  the  hands, 
and  after  stopping,  in  the  direction  of  the  hands.  In  the 
Purkinje  experiment  the  motion  of  objects  is  not  thus  re- 
versed. 

Those  who  try  these  rotational  experiments  should  do  so 
with  caution,  for  the  unpleasant  effects  of  them  sometimes 
last  several  hours. 

Aubert  (Delage),  52,  100  ff. ;  Mach.  Aubert  reprints  Purkinje1  s 
paper  on  dizziness  as  an  appendix  to  the  translation  of  Delage. 

SENSATIONS  OF  PROGRESSIVE  MOTION. 

51.  Progressive  motions,  so  far  as  they  do  not  involve 
rotation,  probably  give  us  combinations  of  sensations  from 
several  different  sources.  The  principle  holds  for  progres- 
sive motions  as  for  rotations,  that  we  perceive  changes  of  rate 
of  motion,  and  not  uniform  motion  ;  as  long  as  the  motion 
remains  uniform  we  can  by  an  effort  of  imagination  conceive 
ourselves  to  be  moving  in  either  direction  or  to  be  standing 
still,  except  for  what  jarring  there  may  be.  The  apparatus 
for  the  study  of  these  phenomena  will  be  found  in  railroad 
trains  and  elevators.  See  also  Mach  for  special  laboratory 
apparatus. 

Aubert  (Delage),  75  ff.;  Mach;  Brown;  Breuer,  283. 


BIBLIOGRAPHY. 

AUBERT  :  Physiologische  Studien  iiber  die  Orientierung,  Tubingen, 
1888,  122.  This  is  a  translation  of  the  paper  of  Delage 
below,  with  full  notes  and  appendix  containing  Purkinje' s  Bul- 
letin von  1825,  Ueber  den  Schwindel.  See  criticism  by  Breuer, 
270  ff. 

OF  THE 
UNIVERSITY 


44         LABORATORY  COURSE  IN  PSYCHOLOGY. 

AYRES  :  A.  A  Contribution  to  the  Morphology  of  the  Vertebrate 
Ear,  with  a  Reconsideration  of  its  Functions,  Journal  of 
Morphology,  VI.,  Nos.  1  and  2,  May,  1892.  Ayres  gives  a 
bibliography  of  nearly  three  hundred  titles,  many  upon  the 
psycho-physiology  of  the  semicircular  canals,  but  not  a  complete 
list. 
B.  The  Ear  of  Man  :  Its  Past,  Present,  and  Future.  Wood's 

Holl  Biological  Lectures,  1890.     An  Abstract  of  A. 
BASTIAN  :   "The  Muscular  Sense;  "  Its  Nature  and  Cortical  Locali- 
sation, Brain,  X.,  1887-88,  1-89.     Discussion  on  the  paper  by 
Ferrier,  Sully,  and  others,  89-137. 
BE  AUNTS  :  Les  Sensations  internes,  Paris,  1889. 
BLOCK  :  Experiences  sur  les  sensations  musculaires,  Revue  Scien~ 
tifique,  XLV.,  No.  10,  1890,  294-301. 

BREUER:  Ueber  die  Function  der  Otolithenapparate,  Pfluger's 
Archiv,  XL VIII.,  1890-91,  195-304. 

BROWN:   A.   On  the  Sense  of  Rotation  and  the  Anatomy  and  Physi- 
ology of  the  Semicircular  Canals  of  the  Internal  Ear,  Journal 
of  Anatomy  and  Physiology,  VIII.,  1874,  327.     Reprinted  by 
Mach,  A,  100. 
B.   On  Sensations  of  Motion,  Nature,  XL.,  1889,  449. 

CHARPENTIER  :  Analyse  experimental  de  quelques  elements  de  la 
sensation  de  poids,  Archives  de .  Physiologic,  Ser.  5,  III.,  1891, 
122-135. 

DELABARRE:  Ueber  Bewegungsempfindungen,  Inaug.  Diss.,  Frei- 
burg, 1891,  111.  The  author  has  also  published  a  portion  of 
the  same  matter  in  Mind,  Ser.  2,  1892,  379-396. 

DELAGE:  Eludes  experimentales  sur  les  illusions  statiques  et  dy- 
namiques  de  direction  pour  servir  a  determiner  les  fonctions 
des  canaux  demicirculaires  de  1'oreille  interne,  Archives  de 
Zool  Exper.,  No.  4,  1886,  535-624  (with  index).  Translated 
by  Aubert  above.  See  also  abstract  of  this  paper  in  Comptes 
rendus,  CIIL,  1886,  749. 

EWALD:  Physiologische  Untersuchungen  iiber  das  Endorgan  des 
Nervus  octavus,  Wiesbaden,  1892.  Gives  a  very  extended 
bibliography. 

FERRIER  :   Functions  of  the  Brain,  London,  1886. 


KIN^STHETIC  AND  STATIC  SENSES.  45 

FULLERTON  AND  CATTELL:  On  the  Perception  of  Small  Differ- 
ences, Publications  of  the  University  of  Pennsylvania,  Philo- 
sophical Series,  No.  2,  Philadelphia,  1892. 

FUNKE:  Der  Tastsinn  und  die  Gemeingefiihle,  Hermann's  Handbuch 
der  Physiologic,  III.,  pt.  2,  289-414. 

GOLDSCHEIDER :  A.  Untersuchungen  iiber  den  Muskelsinn,  Du  Bois- 
Eeymond's  Archiv,  1889,  369  ff.  and  540,  also  Supplement- 
Band,  1889,  141  ff. 

B.  Ueber  den  Muskelsinn  und  die  Theorie  der  Ataxie,  Zeitschrift 
fur  kiln.  Med.,  XV.,  1888-89. 

C.  Ueber  die  Grenzen  der  Wahrnehmung  passiver  Bewegungen, 
Centralblatt  fur  Physiologic,  I.,  1887,  223-225. 

_D.  Ueber  paradoxe  Widerstandsempfindung,  Ibid,  III.,  1889,  90-91. 
HALL  AND  HARTWELL:  Bilateral  Asymmetry  of  Function,  Mind, 

IX.,  1884,  93-109. 

JAMES  :  Principles  of  Psychology,  New  York,  1890. 
KREIDL  :  Beitrage  zur  Physiologic  des  Ohrlabyrinthes  auf  Grand  von 

Versuchen  an  Taubstuminen,  PJlilger^s  Archiv,  LI.,  1891-92, 

119-150. 
LOEB:  A.    Untersuchungen   iiber  den  Fimlraum  der  Hand;  Erste 

Mittheilung,    Gleiche    Fiihlstrecken,  Pfliiger's  Archiv,   XLL, 

1887,  107-127. 
B.  Untersuchungen  iiber  die  Orientirung  im  Fiihlraum  dersHand 

und  im  Blickraum,  Ibid.  XL VI.,  1890,  1-46. 
MACH:  A.  Grundlinien  der  Lehre  von  den  Bewegungsempfindungen, 

Leipzig,  1875,  128.     Gives  bibliography  of  thirty-one  titles. 
B.  Analyse  der  Empfindungen,  Jena,  1886,  69  ff. 
MULLER  UND  SCHUMANN:  Ueber  die  psychologischen  Grundlagen 

der  Vergleichung  gehobener  Gewichte,  Pfluyer's  Archiv,  XLV., 

1889,  37-112. 

MUNSTERBERG:  Die  Willenshandlung,  Freiburg,  1888. 
SCHAEFER:  Die  Erklarung  der  Bewegungsempfindungen  durch  den 

Muskelsinn,  Inaug.  Diss.,  Jena,  1889;  also  an  article  of  similar 

title,  Pfluyer's  Archiv,  LXI.,  1887,  566-640. 
STERNBERG  :  Zur  Lehre  von  den  Vorstellungen  iiber  die  Lage  un- 

serer   Glieder,  Pfliiger'a    Archiv,   XXXVII. ,  1885,  1.     Gives 

bibliography  of  fifty-two  titles. 


46          LABORATORY  COURSE  IN  PSYCHOLOGY. 

VIEBOEDT:  Die  Bewegungsempfindung,  Zeitschrift  fur  Biologie, 
XII.,  1876,  226-240.  See  also  Vierordt's  Physiologic,  5te  Aufl., 
329  ff. 

WALLER:  The  Sense  of  Effort,  Brain,  XIV.,  1891,  179-249, 
433-436,  especially  229  ff.  This  study  is  accompanied  by  a 
bibliography  of  fifty  titles.  Nearly  the  same  portion  of  the 
paper  indicated  here  as  of  special  importance  will  be  found 
under  the  following  title :  Experiments  on  Weight-discrimina- 
tion, Proceedings  of  the  Physiological  Society,  Session  of  Jan. 
30,  1892,  Journal  of  Physiology,  XIII.,  May,  1892. 

WEBER  :  Work  cited  in  bibliography  of  Chap.  I. 

WLASSAK:  Die  statischen  Functionen  des  Ohrlabyrinthes  und  ihre 
Beziehungen  zu  den  Raumempfindungen,  Vierteljahr.  fur  wiss. 
PhilosopUe,  XVI.,  1892,  385-403,  XVII.,  1893,  15-29. 

WUNDT:  Work  cited  in  bibliography  of  Chap.  I.,  3te  Aufl.,  I.,  397  ff., 
II. ,  21  ff.;  4te  Aufl.,  I.,  419  ff. 


53]  SENSATIONS   OF   TASTE  AND  SMELL.  47 


CHAPTER   III. 
Sensations  of  Taste  and  Smell. 

THESE  sensations  are  of  secondary  importance  in  psy- 
chology, and  have  received  a  correspondingly  small  -  share 
of  investigation.  In  subjective  quality  they  seem  to  stand 
midway  between  the  general  senses  mentioned  at  the  end 
of  Chapter  I.  and  the  higher  senses  of  Hearing  and  Vision. 

SENSATIONS  OF  TASTE. 

52.  Tastes   and   Smells.      Much   of   what   is   commonly 
called  taste  is  really  a  combination  of  taste  with  smell  and 
with  touch  in  its  various  forms.     With  the  nostrils  held,  try 
to  distinguish  by  taste  alone  between  small  quantities  of 
water  and  a  weak   solution  of  essence  of  clove  in  water. 
A  discrimination  that  is  easily  possible  with  the  nostrils 
open  is   difficult   or  impossible  with   the   nostrils   closed. 
The  solution  should  not  be  swallowed,  for  then  the  olfac- 
tory region  may  be  reached  from  the  back  of  the  nose. 

53.  Distribution  of  the  Organs  of  Taste,     a.  Using  the 
weaker  taste  solutions,  and  operating  upon  yourself  with  a 
mirror  or  on  another  person,  find  out  as  nearly  as  you  can 
in  what  part  of  the  tongue  the  strongest  sensations  are  pro- 
duced by  each.     Test  the  tip,  the  sides,  the  back,  and  the 
middle,  putting  the  solutions  on  with  a  camel's-hair  brush, 
and  rinsing  the  mouth  as  often  as  necessary.     Try  also  the 
hard  and  soft  palates. 

b.  Dry  the  tongue  with  a  handkerchief,  and  test  the  in- 
dividual fungiform  papillae  with  the  stronger  solutions, 


48         LABORATORY  COURSE  IN  PSYCHOLOGY.         [55 

applying  them  with  fine  camel's-hair  pencils.  It  will  be 
found  possible  to  get  taste  sensations  from  the  single 
papillae,  though  perhaps  not  all  four  from  each.  Einse  the 
mouth  as  needed.  Test  the  surface  of  the  tongue  between 
the  papillae  and  observe  that  no  taste  sensations  follow. 

a.  Rittmeyer;  b.  Oehrwall. 

54.  Minimal  Tastes,    a.  Find  what  is  the  greatest  dilution 
of  the  weaker  solutions  in  which  the  characteristic  tastes  can 
still  be  recognized.    The  same  quantity,  e.g.,  half  ateaspoon- 
ful,  should  be  taken  into  the  mouth  at  each  trial,  and  may 
be  swallowed  with  advantage.     Rinse  the  mouth  thoroughly 
as  required.     The  following   are   the  average  proportions 
found  by  Bailey  and  Nichols  for  male  observers :  Quinine, 
1 : 390  000 ;  Sugar,  1 : 199 ;  Salt,  1  : 2240  5  for  Sulphuric  Acid, 
which  they  used  instead  of  Tartaric,  the   proportion  was 
1  : 2080. 

b.  The  intensity  of  the  sensation  and  the  greatest  dilution 
still  tastable  depend  on  the  number  of  taste  organs  stimu- 
lated.    Take  a  portion  of  one  of  the  solutions  of  just  tast- 
able strength,  found  in  a,  add  an  equal  quantity  of  water, 
and  take  a  large  mouthful  of  the  mixture.     The  character- 
istic taste  will  still  be  perceived,  perhaps   more  strongly 
than  before. 

a.  Bailey  and  Nichols,  A-  Lombroso  und  Ottolengm;  Camerer,  A. 
b.  Camerer,  B. 

55.  Discriminative    Sensibility  for  Taste.     For  a  rough 
determination,  test  with  solutions  of  sugar,  taking  first  a 
small  quantity  of  the  standard  20%  solution,  then  an  equal 
quantity  (the  equality  is  important)  of  one  of  the  weaker 
solutions,  or  first  one  of  the  weaker  and  then  the  standard, 
until  a  solution  is  found  that  is  just  recognizably  different 
from  the  standard.     Make  this  determination  several  times. 
The   excess   of   sugar   in   the   standard   solution   over  the 


57]  SENSATIONS   OF   TASTE  AND  SMELL.  49 

amount  in  the  solution  just  observably  weaker,  set  in  a  ratio 
to  the  total  percentage  of  sugar  in  the  standard,  measures 
the  sensibility.  Some  experimenters  may  be  able  to  dis- 
tinguish the  18%  from  the  20%  solution;  their  sensibility 
would  then  be  expressed  by  the  ratio  2  :  20. 
Keppler. 

56.  Electrical  Stimulation,     a.  Using  a  constant  current, 
from  a  single  Grenet  cell,  for  example,  and  two  small  zinc 
electrodes,  one  applied  to  the  inner  surface  of  the  under  lip 
and  the  other  to  the  tongue,  notice  the  sour  taste  at  the 
positive  pole  and  the  alkaline  at  the  negative. 

Yon  Vintschgau,  181  ff. ;  Oehrwall;  Hermann. 
SENSATIONS  OF  SMELL. 

57.  Minimal   Odors.      The   keenness   of   smell  may  be 
tested  with  dilute  solutions  of  odorous  substances  or  with 
the   olfactometer. 

a.  Test  with  solutions.  Pour  small  quantities  of  the 
solutions  of  oil  of  cloves  into  little  wide-mouthed  bottles, 
filling  each  to  about  the  same  height.  Mark  all  in  an  in- 
conspicuous manner.  Set  the  bottles  a  foot  apart  on  a 
table  in  a  place  where  there  is  moderate  circulation  of 
air,  in  the  order  of  the  strength  of  their  solutions,  be- 
ginning with  the  water  and  following  with  the  weakest  so- 
lution and  so  on.  Require  the  subject  to  smell  of  the 
bottles  in  succession  without  lifting  them  from  the  table, 
beginning  with  the  water,  and  to  indicate  that  in  which  he 
first  recognizes  a  characteristic  odor.  If  the  solutions  stand 
for  any  length  of  time  where  they  are  subject  to  evaporation, 
it  will  be  safer  to  prepare  fresh  ones  before  undertaking  a 
new  test.  Other  precautions  will  suggest  themselves,  such 
as  the  use  of  similar  bottles,  and  care  in  filling  them  that 
none  of  the  solution  is  left  clinging  near  the  mouth. 


50          LABORATORY  COURSE  IN  PSYCHOLOGY.         [59 

b.  Test  with  the  olfactometer.  Test  the  sides  of  the  nose 
separately.  Push  the  odor-tube  on  till  its  end  is  flush  with 
that  of  the  glass  tube,  insert  the  bent  end  of  the  latter  into 
the  nostril,  and  gradually  lengthen  the  exposed  surface 
of  the  odor-tube  till  its  odor  is  just  discernible.  Note  in 
millimeters  the  length  exposed. 

a.  Bailey  and  Nichols,  B\  Lombroso  und  Ottolenghi ;  Savelieff; 
6.  Zwaardemaker,  A  and  C. 

58.  Discriminative    Sensibility   for    Odors.     Using    the 
double   olfactometer  with   both   odor-tubes  drawn  out  far 
enough  to  give  an  unmistakable  odor,  but  not  too  strong  a 
one,  say  both  drawn  out  5  cm.,  find  how  far  one  or  the  other 
must  be  drawn  out  (or  pushed  back)  to  make  the  odor  which 
it  gives  just  observably  stronger  (or  weaker)  than  that  of 
the  other.     The  test  should  be  made  with  the  sides  of  the 
nose  separately  (there  is  frequently  a  difference  in  sensi- 
tiveness between  the  two  sides,  due  to  mechanical  obstruc- 
tion or  other  cause),  unless  for  some  reason  a  bilateral  form 
of  experiment  is  desirable.     Try  a  number  of  times,  in  half 
the  tests  smelling  the  weaker  before  the  stronger,  and  in 
half  the  stronger  before  the  weaker,  but  be  careful  to  avoid 
fatigue. 

59.  Fatigue  of  Smell,     a.  Hold  a  piece  of  camphor  gum 
to   the   nose,   and   smell  of   it  continuously,  breathing   in 
through  the  nose  and  out  through  the  mouth,  for  five  or  ten 
minutes.     A  very  marked  decrease  in  the  intensity  of  the 
sensation  will  be  observed,  reaching  perhaps  even  to  com- 
plete loss  of  the  odor. 

b.  It  is  important,  however,  to  observe  that  fatigue  for  one 
substance  does  not  cause  obtuseness  for  all  other  substances, 
though  it  does  for  some.     Smell  of  some  essence  of  cloves 
and  of  some  yellow  wax,  then  fatigue  for  camphor  as  in  a, 
and  smell  of  the  essence  of  cloves  and  of  the  wax  again. 


60]  SENSATIONS   OF   TASTE  AND  SMELL.  51 

The  odor  of  the  wax  will  probably  be  fainter,  that  of  the 
essence  of  cloves  unaffected. 
Aronsohn. 

60.  Combination  of  Odors.  Experiment  with  the  olfac- 
tometer  on  one  side  of  the  nose  as  follows.  Hold  against 
the  end  of  the  rubber  odor-tube  another  odor-tube  of  wax 
(partly  covered  on  the  inside  by  a  glass  tube  of  the  same 
size  as  that  used  in  the  olfactometer),  in  such  a  way  that 
the  air  must  pass  through  both  to  reach  the  nose.  Then 
gradually  increase  the  length  of  the  rubber  tube  exposed 
till  the  odor  of  the  wax  is  no  longer  perceived.  If  the 
experiment  is  carefully  performed,  a  point  may  be  found 
where  the  odors  nearly  balance.  If  the  rubber  is  length- 
ened beyond  this  point,  its  odor  overpowers  that  of  the 
wax;  if  it  is  shortened,  it  is  overpowered  by  that  of  the 
wax.  A  mixture  of  the  odors  in  which  both  can  be  detected 
is  difficult  to  find.  Care  should  of  course  be  taken  to  avoid 
fatigue. 

A  similar  balance  of  odors  was  found  by  Zwaardemaker 
when  the  double  olfactometer  was  used  and  the  two  sides  of 
the  nose  received  separate  stimuli. 

Zwaardemaker,  B. 


BIBLIOGRAPHY. 

ARONSOHN:  Experimentelle  Untersuchungen  zur  Physiologic  des 

Geruchs,  Du  Bois-EeymonV  s  Archiv,  1886,  321-357. 
BAILEY  AND  NICHOLS:  A.   The  Delicacy  of  the  Sense  of  Taste, 

Nature,  XXXVII.,  1887-88,  557;  also  Science,  1888,  145. 
B.   The  Sense  of  Smell,  Nature,  XXXV.,  1886-87,  74. 
CAMERER:  A.  Die  Grenzen  der  Schmeckbarkeit  von  Chlornatrium 

in  wasseriger  Losung,  Pfluger^s  Archiv,  II.,  1869,  322. 
B.   Die  Methode  der  richtigen  und  falschen  Falle  angewendet  auf 
den  Geschmackssinn,  Zeitschrift  fur  Biologic,  XXI.,  570. 


52         LABORATORY  COURSE  IN  PSYCHOLOGY. 

CORIN:  Action  des  acides  sur  le  gout,  Archives  de  Biologic,  VIII., 
1888,  fasc.  1,  121-138. 

GOLDSCHEIDER  UND  SCHMIDT  :  Bemerkungen  iiber  den  Geschmack- 
sinn,  Centralblatt  fur  Physiologic,  IV.,  1890,  10-12. 

HAYCRAFT:  The  Nature  of  the  Objective  Cause  of  Sensation;  Taste, 
Brain,  X.,  1887,  145-163;  Smell,  Ibid.,  XL,  1888-89,  166-178. 

HERMANN:  Beitrage  zur  Kenntniss  des  elektrischen  Geschmacks, 
nach  Versuchen  von  Laserstein,  Pfliiger's  Archiv,  XLIX.,  1891, 
519^538. 

HOWELL  AND  KASTLE :  Note  on  the  Specific  Energy  of  the  Nerves 
of  Taste,  Studies  from  the  Biological  Laboratory,  Johns  Hopkins 
University,  IV.,  No.  1. 

KEPPLER:  Das  Unatrscheidungsvermogen  des  Geschmacksinnes  fiir 
Concentrationsdifferenzen  der  schmeckbaren  Korper,  Pfluger's 
Archiv,  II.,  1869,  449. 

LOMBROSO  UND  OxTOLENGHi:  Die  Sinne  der  Verbrecher,  Zeitschrift 
fur  Psychologic,  II. ,  1891,  342-348. 

OEHRWALL:  Untersuchungen  iiber  den  Geschmackssinn,  Scandinav. 
Archiv  f.  PhysioL,  II.,  1890,  1-69;  see  also  abstract  by  the 
author  in  the  Zeitschrift  f.  Psych.,  I.,  1890,  141. 

PASSY:  Sur  les  minimums  perceptibles  de  quelques  odeurs,  Comptes 
rendus,  CXIV.,  1892,  306. 

RAMSEY:  On  Smell,  Nature,  XX VI.,  1882,  187. 

RITTMEYER:  Geschmackspriifungen,  Inaug.  Diss.,  Gottingen,  1885. 

SAVELIEFF:  Untersuchung  des  Geruchsinnes  zu  klinischen  Zwecken, 
Neurologisches  Centralblatt,  XII.,  1893,  340-345. 

SHORE:  A  Contribution  to  our  Knowledge  of  Taste  Sensations, 
Journal  of  Physiology,  1892,  191-217. 

VON  VINTSCHGAU:  Physiologic  des  Geschmackssinns  und  des  Ge- 
ruchssinns,  Hermann's  Handbuch  der  Physiologic,  III.,  pt.  2, 
143-286.  For  a  general  account  of  the  physiology  and  psychol- 
ogy of  taste  and  smell  to  1880,  and  references  to  the  earlier 
literature,  see  this  work. 

WUNDT:  Work  cited  in  bibliography  of  Chap.  I.,  3te  Aufl.,  I.,  411  ff.; 
4te  Aufl.,  438  ff. 


SENSATIONS   OF   TASTE  AND  SMELL.  53 

ZWAAKDEM AKEB  :  A.  Die  Bestimmung  der  Gerachscharfe,  Berliner 
kiln.  Wochenschrift,  XXV.,  1888,  No.  47,  950  (abstract  of  the 
same,  British  Med.  Journal,  1888,  ii.  1295);  also  Lancet, 
London,  1889,  i.  1300. 

B.  Compensation  von  Geruchen  mittelst  des  Doppelreichmessers, 
Fortschritte  der  Medicin,  VII.,  1889,  721  ff. 

C.  Sur  la  norme  de   1'acuite    olf active  (olfactie),  Archives  neer- 
landaises,  XXV,,  131-148. 

J).  La  mesure  des  sensations  olf  actives  et  Tolfactometre,  Revue 
scientifique,  1889,  ii.  810-812.  Extract  from  the  Archives 
neerlandaises. 


54         LABORATORY  COURSE  IN  PSYCHOLOGY.         [61 


CHAPTEE   IV. 
Sensations  of  Hearing. 

IN  these  experiments  a  little  knowledge  of  the  physics  of 
sound  is  presupposed  —  as  much  as  would  be  given  in  an 
elementary  course  in  physics.  A  very  little  knowledge  of 
musical  notation  is  also  required,  but  hardly  more  than 
everybody  has.  No  special  musical  skill  is  needed  except 
in  Exs.  70  and  93  b.  It  is  also  the  author's  belief  that  most 
persons  calling  themselves  "  unmusical,"  however  truly 
they  may  rate  themselves  as  performers,  are  very  much 
in  error  as  to  their  ability  to  discriminate  musical  sounds. 
The  greatest  difficulty  in  some  of  these  experiments  will  be 
found  in  the  continuous  intrusion  of  outside  sounds,  and 
some  even  may  have  to  be  tried  at  night. 

SOUNDS  IN  GENERAL. 

61.  Minimal  Sounds,  a.  Experiment  in  a  large  room  as 
free  as  possible  from  noise.  Let  the  subject  be  seated  with 
his  side  toward  the  experimenter,  his  eyes  closed,  and  his 
ear  upon  the  other  side  plugged  with  cotton.  Let  the 
experimenter  then  find  what  is  the  greatest  distance  at 
which  the  subject  can  still  hear  the  tick  of  a  watch  held 
at  the  level  of  his  ear  and  on  the  prolongation  of  the  line 
joining  the  two  ears.  This  is  easily  done  with  sufficient 
accuracy  by  drawing  a  chalk  line  on  the  floor,  marking  off 
feet  or  meters  and  fractions  upon  it,  and  estimating  by  eye 
the  point  of  the  line  directly  under  the  watch.  Try  several 
times  for  each  ear,  both  when  the  watch  is  being  brought 


63]  SENSATIONS   OF  HEARING.  55 

toward  the  ear  and  when  it  is  being  carried  away.  The 
experimenter  should  from  time  to  time  cover  the  watch  with 
his  hand  to  discover  whether  or  not  the  subject  really  hears, 
or  is  under  illusion.  For  normal  ears  the  distance  found 
may  vary  from  2.5  m.  to  4.5  m.,  and  may  even  rise  to  as 
much  as  9  m. 

b.  The  subject  should  notice  in  this  experiment  the  very 
marked  intermittences  of  the   sound  when  just  upon  the 
limit  of  audibility.     It  will  for  a  few  seconds  be  heard  dis- 
tinctly, and  a  few  seconds  later  will  as  distinctly  not  be 
heard. 

c.  Faint  sounds  are  apt  to  be  underestimated.     Place  a 
sounding   tuning-fork   on  the  head  and  let  the  sound  die 
away  to  almost  complete  extinction ;  then  remove  it.     The 
drop  to  complete  silence  will  often  seem  larger  than  the 
apparent  intensity  of  the  tone  would  justify. 

On  a,  von  Bezold,  A;  on  6,  Urbantschitsch,  A]  Lange;  Miinster- 
berg,  A',  on  c,  Stumpf,  I.,  388,  who  quotes  from  Fechner. 

62.  Discriminative  Sensibility  for  Intensity  of   Sounds. 
Exact  experiments  on  this  topic  are  difficult  to  make,  be- 
cause of  the  very  great  difficulty  of  determining  objectively 
the  intensity  of  the  sounds  used.     A  rough  determination 
can  easily  be  made,  however,  with  the  sound  pendulum  (see 
chapter   on    apparatus).      Choose   a   medium   sound   as   a 
standard,  and  by  the  Method  of  Just  Observable  Difference 
explained  under  Ex.  24,  find  a  sound  that  is  just  recogni- 
zably different  from  it.     The  discriminative  sensibility  is 
very  much  finer,  apparently,  when  the  question  is  not  one 
of  recognizing  a  difference,  but  of  locating  a  sound  as  right 
or  left  of  the  median  plane.     Cf.  Ex.  101  and  Rayleigh. 

Wimdt,  3te  Aufl.,  I.,  364  ff. ;  4te  Aufl.,  I.,  360  fif. ;  Stumpf,  I.,  345  ff. 

63.  Auditory  Fatigue,     a.  Cause   an   assistant  to  strike 
once   with   a   hammer   on  the  floor,  or  to  clap  his  hands. 


56          LABORATORY  COURSE  IN  PSYCHOLOGY.         [63 

With  the  ears  open  a  single  sound,  or  at  most  a  single  sound 
and  transient  echoes  are  heard.  If,  however,  the  ears  are 
kept  closed  with  the  fingers  till  half  a  second  or  more  after 
the  stroke  (the  time  may  easily  be  fixed  by  rapid  counting), 
the  fainter  echoes  will  be  heard  on  the  opening  of  the  ears, 
like  a  new  stroke.  In  the  first  case,  fatigue  from  the  origi- 
nal sound  deadens  the  ears  to  the  fainter  echoes,  though 
they  may  still  be  heard  by  attentive  listening;  in  the 
second  case  they  are  more  strongly  heard  because  the  closed 
ears  are  unfatigued.  The  sound  produced  by  the  simple 
opening  of  the  ears  without  any  objective  stroke  will  be  less 
if  the  finger  is  not  put  into  the  ears,  but  presses  the  tragus 
back  upon  the  opening. 

b.  Strike  a  tuning-fork,  press  the  stem  firmly  upon  the 
mastoid  process,  or  the  crown  of  the  head,  and  hold   it 
there   till   the   tone   is   no   longer  heard.     Then   instantly 
remove  it,  and  after  a  second  or  two  replace  it  upon  the 
same   spot,  taking  pains  to  press  no  harder  than   before. 
The  fork  will  be  heard  again  sounding  faintly.     The  experi- 
ment may  not  succeed  at  first,  but  a  few  trials  should  not 
fail  to  show  the  effect. 

c.  Insert  in  the  openings  of  the  ears  the  ends  of  a  rubber 
tube.     Strike  a  tuning-fork  and  set  it  upon  the  tube  at  such 
a  point  that   it   sounds    equally  intense   to  the  two  ears. 
The  sound  will  then  probably  appear  to  be  located  in  the 
head  midway  between  the  ears  —  at  least  not  nearer  one 
than  the  other.     After  a  few  seconds  strike  the  tuning-fork 
again,  pinch  the  tube  on  one  side,  say  the  left,  so  as  to  shut 
off  the  sound  from  the. ear  on  that  side,  set  the  tuning-fork 
at  the  proper  place  on  the  tube  and  keep   it  there  till  the 
sound  has  become  rather  faint.      Then  allow  the  pinched 
tube  to  open,  and  notice  that  the  sound  is  now  stronger  on 
the  left  than  the  right  and  apparently  located  on  the  left. 
Try  the  experiment  in  reverse  form,  pinching  the  tube  on 
the  right. 


64]  SENSATIONS    OF  HEARING.  57 

Cf.  later  experiments  on  the  analysis  of  compound  tones 
by  the  fatigue  method,  Ex.  89  c. 

Stumpf,  I.,  360-363.  On  a,  Mach;  on  6,  Corradi;  on  c,  Urbant- 
schitsch,  B. 

64.  Inertia  of  the  Auditory  Apparatus,  a.  Inertia  tend- 
ing to  keep  the  auditory  apparatus  out  of  function  can  be 
demonstrated  as  follows.  Place  the  ends  of  a  rubber  tube 
in  the  ears,  and  set  upon  the  middle  of  it  a  low  tuning-fork 
sounding  as  faintly  as  possible.  Notice  that  the  sound  does 
not  reach  its  maximum  intensity  for  an  appreciable  length 
of  time  ;  if  the  fork  is  barely  audible,  this  may  be  as  much 
as  a  second  or  two.  Be  careful  not  to  increase  the  pressure 
of  the  fork  nipon  the  tube  after  first  setting  it  on,  for  that 
will  produce  an  objective  strengthening  of  the  tone  ;  and 
allow  an  interval  of  several  seconds  between  the  tests  so 
that  the  auditory  apparatus  may  again  come  completely  to 
rest.  A  tuning-fork  that  will  preserve  these  minimal  vibra- 
tions for  some  seconds,  and  complete  freedom  from  distract- 
ing noises,  will  be  found  necessary  for  success. 

b.  Inertia  tending  to  keep  the  auditory  apparatus  in  func- 
tion (positive  auditory  after-images)  can  be  demonstrated  as 
follows.  Fasten  upon  the  front  of  a  rather  solid  pendulum 
a  small  tuning-fork,  so  that  it  shall  project  forward  at  right 
angles  to  the  pendulum  bar  and  the  tines  of  the  fork  shall 
be  vertically  one  above  the  other.  On  the  three  arms  of  a 
Y-tube  attach  three  pieces  of  small  rubber  tubing,  say  quar- 
ter inch  outside  measurement.  Those  fitting  on  the  upper 
arms  of  the  Y  should  be  of  the  same  length,  that  fitting  upon 
the  stem  may  be  of  any  convenient  length.  Insert  the  free 
end  of  the  last  mentioned  tube  in  the  outer  passage  of  the 
ear,  and  hold  the  tips  of  the  other  tubes  about  half  an  inch 
apart,  open  end  upward,  in  such  a  way  that  the  tip  of  the 
tuning-fork,  as  the  pendulum  swings,  will  pass  close  over 
them.  Strike  the  fork  with  a  small  rubber  hammer  as  the 


58          LABORATORY  COURSE  IN  PSYCHOLOGY.          [65 

pendulum  swings  and  notice  the  sound  produced  by  the 
fork  as  it  passes  the  ends  of  the  tube.  If  a  single  continu- 
ous sound  is  heard,  separate  the  tubes  a  little ;  if  a  double 
sound  is  heard,  bring  them  together ;  and  thus  by  shifting 
them  back  and  forth  find  the  place  where  the  sounds  just 
fuse  into  one.  The  auditory  disturbance  occasioned  by  the 
first  pulse  of  sound  outlasts  the  interval  between  the  two, 
and  blends  with  the  second.  Move  the  pendulum  slowly 
over  the  end  of  one  tube  and  then  of  the  other,  meantime 
pinching  the  tube  over  which  the  fork  is  sounding,  to  con- 
vince yourself  that  the  tone  is  not  heard  at  substantially 
the  same  instant  in  both  tubes.  It  is  possible  from  the 
rate  of  the  pendulum  and  the  separation  of  the  tubes  to  find 
approximately  the  length  of  time  through  which  the  sensa- 
tion persists. 

c.  Sometimes  it  is  possible  to  get  more  lasting  after-im- 
ages and  even  those  that  are  recurrent.  Try  with  a  tuning- 
fork  struck  and  held  a  few  seconds  before  one  ear.  Stop 
the  fork  by  touching  it,  without  removing  it  from  the  ear. 
The  after-image  is  not  very  easy  to  observe;  the  lowest 
degree  of  it  seems  to  be  the  transforming  of  faint  outer 
noises  into  something  qualitatively  like  the  tone  heard,  or 
perhaps  a  selection  of  certain  of  those  noises.  The  usual 
interval  between  the  stimulus  and  the  after-image  is  under 
fifteen  seconds.  The  number  of  recurrences  of  the  after- 
image differs  in  different  subjects ;  for  Stumpf,  they  seem 
to  come  by  preference  in  the  uiistimulated  ear. 

Stumpf,  I.,  211  if,  278;  Urbantschitsch,  C.  For  methods  of  dem- 
onstration permitting  more  accurate  measurement  of  the  persistence 
of  tone,  see  Urbantschitsch,  C,  and  Mayer,  A. 

65.  Noise.  Whether  or  not  there  is  a  distinctive  sensa- 
tion of  noise  different  from  that  of  a  mass  of  short,  disso- 
nant, and  irregularly  changing  tones,  is  yet  under  debate. 
A  little  attention  to  the  noises  constantly  occurring,  espe- 


68]  SENSATIONS    OF  HEARING.  59 

cially  to  their  pitch,  will  easily  convince  the  observer  that  a 
tonal  element  is  present.  This  is  striking  when  resonators 
(cf.  notes  on  apparatus  for  simultaneous  tones)  are  used, 
for  they  pick  out  and  prolong  somewhat  the  tones  to  which 
they  correspond,  but  they  are  not  indispensable.  On  the 
other  hand,  attention  to  musical  tones  will  often  discover 
the  presence  of  accompanying  noises. 

Wundt,  3te  Aufl.,  I.,  420;  4te  Aufl.,  I.,  447  f ;  Stumpf,  II.,  497- 
515;  Briicke;  Exner;  Mach,  B,  117. 

66.  Silence.     When   circumstances   promise   absence   of 
external  sounds,  notice  that  many  are   still   present   and 
distinct,  though  faintly  heard.     Notice  also  the  pitch  and 
changing  character  of  the  subjective  sounds  to  be  heard. 
Our  nearest  approach  to  the  experience  of  absolute  stillness 
is  this  mass  of  faint  inner  and  outer  sensations. 

Preyer,  A,  67-72;  Stumpf,  I.,  380  ff. 

SINGLE  AND  SUCCESSIVE  TONES. 

67.  Highest  Tones.     With  the  apparatus  at  hand  for  the 
purpose,  find  what  is  the  highest  audible  tone  ;  i.  e.,  if  the 
cylinders  are  used,  the  shortest  cylinder  which  still  gives  a 
ringing  sound  when  struck  with  the  hammer,  or  if  the  whis- 
tle is  used,  the  closest  position  of  the  plunger  at  which  a 
tone  can  still  be  heard  beside  the  rush  of  air.     If  a  number 
of  persons  are  tested,  it  is  not  improbable  that  some  will  yet 
hear  the  tone  after  it  has  become  inaudible  for  the  rest. 

Same  references  as  Ex.  68. 

68.  Lowest  Tones.     If  low-pitched  tuning-forks  or  other 
vibrators  are  at  hand,  find  what  is  the  slowest  rate  of  vibra- 
tion that  can  yet  be  perceived  as  a  tone.     In  some  physio- 
logical laboratories  electric  tuning-forks  or  interrupters  may 
be  found  that  have  vibration  rates  of  twenty-five  per  second. 
Low  tones  can  be  heard  from  these,  though  they  have  many 


60          LABORATORY   COURSE  IN  PSYCHOLOGY.          [69 

overtones.  The  latter  can  be  partly  damped  by  touching  the 
tines  midway  of  their  length  with  the  finger,  and  partly 
avoided  by  bringing  the  ear  not  to  the  free  end,  but  to  a 
point  somewhat  nearer  the  handle.  The  determination  of 
the  lower  limit  of  audible  pitch  is  difficult  and  uncertain 
because  of  the  great  difficulty  which  observers,  even  those 
of  trained  ear,  find  in  distinguishing  these  lowest  tones  from 
the  next  higher  octaves.  The  general  character  of  these 
deep  tones  can  be  demonstrated  with  sufficient  clearness 
upon  the  contra  octave  (C-^-C)  of  a  church  organ,  if  one  is 
accessible  and  tuning-forks  are  lacking. 

Von  Bezold,  JB;  Wundt,  3te   Aufl.,  L,  423;  4te  Aufl.,  I.,  450; 
Preyer,  A  and  D;  Stumpf,  I.,  263,  II.,  551. 

69.    Some  Characteristics  of  High  and  Low  Tones. 

a.  High  tones  are  smoother  than  low  tones.     This  is  clear 
with  almost  all  tones  used  in  music,  and  particularly  so  with 
those  of  reed  instruments.     The  roughness  of  low  tones  is 
largely  due  to  the  beating  of  their  partials  among  them- 
selves (see  Exs.  86  if.  and  79  if.)  and  even  with  the  funda- 
mental tones ;  the  high  tones  having  fewer  audible  partials 
are  freer  from  it.     Play  the  scale  of  any  instrument  from  its 
lowest  to  its  highest  tone,  or  sing  the  ascending  scale.    The 
difference  of  roughness  is  observable  also  with  simple  tones, 
but  only  at  lower  pitches,  and  is  even  there  less  marked. 

b.  In  spite  of  the  generally  accepted  fact  that  high  tones 
produce  a  more  intense  sensation  than  low  tones  of  equal 
physical  energy,  high  tones  are  more  readily  suppressed  by 
stronger  lower  tones  than  vice  versa.     Place  an   ordinary 
clock  at  a  distance  of  a  few  feet  and  hold  close  before  the 
ear  a  watch.     When  the  watch  is  near  the  ear  all  the  ticks 
will  be  heard.     As  it  is  gradually  removed,  a  position  can 
be  found  where  the  watch-tick  that  coincides  with  the  clock- 
tick  will  be  suppressed.     When  both  make  an  equal  number 


69]  SENSATIONS    OF  HEARING.  61 

of  ticks  to  a  second,  and  one  gains  a  little  on  the  other, 
there  will  occur  periods  in  which  no  watch-ticks  are  heard, 
and,  alternating  with  them,  periods  in  which  all  are  heard. 
If  the  watch  beats  oftener  than  the  clock  and  both  run  at 
the  same  rate,  a  single  watch-tick  will  be  lost  at  regular  in- 
tervals. When  the  clock  is  removed,  all  the  ticks  of  the 
watch  can  easily  be  heard  at  the  distance  used.  The  phe- 
nomenon can  be  observed  when  the  watch  is  on  the  opposite 
side  of  the  head  from  the  clock.  To  demonstrate  weakness 
of  high  tones  in  suppressing  lower  tones,  sound  together  a 
large  and  a  small  tuning-fork  on  their  resonance  cases,  e.  g., 
c  and  6*",  a",  or  £",  sounding  the  first  very  faintly  and  the 
second  as  loudly  as  possible.  The  first  will  still  be  heard 
even  when  the  second  is  brought  close  to  the  ear.  In  this 
connection  compare  the  difficulty  of  analyzing  the  compound 
tones  in  Exs.  86  if.,  also  Exs.  83  b  and  84. 

c.  Some  high  tones  are  particularly  strengthened  by  the 
resonance  of  the  outer  passage  of  the  ear.     These  generally 
lie  between  c4  and  c5,  and  give  to  the  tones  of  this  octave  a 
superior  strength  and  ear-piercing  quality.     They  may  be 
demonstrated  easily  with  a  small  piston  whistle.     Find  by 
adjustment  of  the  piston  the  point  at  which  the  tone  is  most 
piercing.     Insert  in  the  outer  ends  of  the  ear-passages  bits 
of  rubber  tubing  half  an  inch  long  (which  will  change  the 
resonance  of  the  passages,  making  them  responsive  to  a 
lower  tone)   and  sound  the  whistle  again.     The  piercing 
quality  will  be  gone  and  the  tone  appear  decidedly  Aveaker. 
Remove  the  bits  of  tubing  and  sound  the  whistle  as  before  ; 
Lhe  original  quality  and  intensity  reappear. 

d.  Very  closely  associated  with  the  pure  tonal  sensations 
are  certain  of  a  spatial  quality.     Compare  in  this  respect 
the  sensations  of  the  tones  observed  in  c  above ;  or,  better 
still,  those  of   Ex.  67  with  those  of  Ex.  68,  or  any  other 
deep  tones.     Play  the  scale  through  the  complete  compass  of 
any  instrument,  keeping  this  quality  in  mind. 


62          LABORATORY  COURSE  IN  PSYCHOLOGY         [?0 

e.  Under  certain  conditions,  low  tones  seem  to  be  located 
in  the  head,  high  tones  outside  of  it.  Close  the  ears  with 
the  fingers  and  have  an  assistant  strike  a  low  tuning-fork 
(e.g.,  50  vibrations  per  sec.),  and  set  the  stem  of  it  upon  the 
crown  of  the  head ;  notice  the  location.  Try  the  same 
with  a  high  fork. 

/.  The  emotional  shading  of  tones  changes  with  their 
pitch.  Recall  the  descriptive  terms  used  :  Deep,  low, 
bright,  sharp,  acute.  Play  the  scale,  and  judge  of  the  ap- 
propriateness of  these  terms  to  match  the  shades  of  feeling 
that  mark  the  tones  of  low,  middle,  and  high  pitch,  distin- 
guishing those  that  refer  to  pitch  from  those  enumerated 
in  Ex.  90,  which  refer  to  timbre. 

Stumpf,  I.,  202-220,  II.,  56-59,  227;  also  Mach,  #,  120  ff.  On  6, 
Mayer,  B\  on  c  and  /,  Helmholtz,  116,  179,  and  69  ff. ;  on  tf,  James, 
II.,  134  ff.;  on  e,  Kessel. 

70.  Recognition  of  Absolute  Pitch,  a.  This  experiment 
gives  accurate  results  only  with  those  of  very  decided 
musical  skill,  but  it  may  be  tried  with  any  subject  that 
knows  the  names  of  the  notes.  Strike  various  notes  in  dif- 
ferent parts  of  the  scale  of  the  instrument  and  require  the 
subject  to  name  the  note  given.  Record  the  note  struck 
and  the  subject's  answer.  He  should  be  seated  with  his 
back  toward  the  experimenter,  or  should  keep  his  eyes 
closed. 

b.  Pitch  differences  in  the  perceptions  of  the  two  ears. 
The  same  tone,  heard  first  with  one  ear  and  then  with  the 
other,  seems  to  many  observers,  even  professional  musicians, 
somewhat  different  in  pitch.  Take  two  small  rubber  tubes 
of  equal  size  and  length  (e.  g.,  quarter  inch  tubes,  two  feet 
long),  place  an  end  of  one  in  the  right  ear,  an  end  of  the 
other  in  the  left,  and  bring  the  free  ends  near  together  on 
the  table.  Then  have  an  assistant  strike  a  tuning-fork  and 


72]  SENSATIONS   OF  HEARING.  63 

present  it  alternately  to  the  ends  of  the  tubes.  The 
difference  between  the  two  ears  is  said  to  vary  more  or  less 
from  day  to  day  and  to  be  different  in  amount  for  tones  of 
different  pitch.  Such  differences  may  be  observed  by  the 
unmusical. 

Stumpf,  L,  305-313,  also  II.,  index,  Hohenurteile,  for  experiments 
on  trained  musicians;  von  Kries,  J5;  on  6,  Stumpf,  II.,  319  f. 

71.  Just  Observable  Difference  in  Pitch.    Test  as  follows 
with  the  set  of  mistuned  forks.     Let  the  subject  pick  out 
from  the  mistuned  forks  that  which  sounds  to  him  just 
noticeably  different  from  the  normal  fork,  striking  and  hold- 
ing them  successively  (never  simultaneously)  over  a  reson- 
ance bottle.     If  all  of  them  seem  more  than  just  observably 
different,  let  him  put  the  riders  on  the  one  that  is  next 
higher,  and    gradually  lower   the   pitch   by  sliding  them 
toward  the  ends  of  the  fork  till  the  two  forks,  heard  suc- 
cessively, are  just  different  and  no  more.     The  experimenter 
may  then  determine  the  error  of  the  subject  in  vibrations 
per  second  approximately  by  counting  the  number  of  beats 
produced   by   the   forks   when   sounded   together.     If  the 
number  of  beats  per  second  is  less  than  2  or  more  than  6, 
it  will  be  best  to  get  the  difference  in  pitch  with  some  other 
of  the  forks  first,  so  as  to  avoid  too  slow  or  too  rapid  count- 
ing, and  from  that  to  arrive  at  the  difference   from   the 
standard  fork.     Kepeat  the  test  several  times,  sometimes 
sounding  the  standard  fork  first,  and  sometimes  that  to  be 
compared  with  it,  and  average  the  result.     Take  care  to 
avoid  fatigue.     This  experiment  will  not  be  refined  enough 
for  testing  those  of  keen  musical  ear. 

Preyer,  A,  26  if.,  D,  64;  Stumpf,  L,  296-305;  Luft. 

72.  Differences  in  Pitch  that  are  Just  Eecognizable  as 
Higher  or  Lower.     It  is  easier  to  recognize  a  difference 
than  to  tell  its  direction.     Experiment  as  in  Ex.  71,  but 


64          LABORATORY  COURSE  IN  PSYCHOLOGY.          [74 

require  the  subject  this  time  to  pick  out  and  adjust  a  fork 
that  is  just  observably  sharper  or  natter  than  the  standard. 
Preyer,  A,  28,  36.     For  experiments  on  extremely  unmusical  sub- 
jects, see  Stumpf,  I.,  313-335. 

73.  Number  of  Vibrations  Necessary  to  Produce  a  Sensa- 
tion of  Pitch.     Arrange  an  apparatus  for  blowing   soap- 
bubbles  with  a  mixture  of  hydrogen  and  air.     Blow  bubbles 
of  different  sizes  and  touch  them  off  with  a  match,  either  in 
the  air,  or  (if  proper  precaution  is  taken  to  prevent  the  igni- 
tion of  the  mixed  gases  in  the  vessel  and  any  resonance  in 
the   pipe),  while   still  hanging.     The   explosion   of  these 
bubbles  is  supposed  to  produce  a  single  sound  wave.     The 
pitch  of  the  sounds  produced  cannot  be  accurately  givenj 
but  the  report  of  the  large  bubbles  is  distinctly  deeper  than 
that  of  the  small  ones. 

Briicke;  Cross  and  Maltby;  Herroun  and  Yeo. 

74.  The  Apparent  Pitch  of  Tones  is  Affected  by  their 
Quality.     Tones  of  dull  and  soft  character  seem  lower  in 
pitch  than  those  that  are  brighter  and  more  incisive.     Re- 
quire the  subject  to  pick  out  on  some  stringed  or  reed  instru- 
ment the  tone  corresponding  to  that  produced  by  blowing 
across  the  mouth  of  a  medium-sized  bottle.     Too  low  a  note 
at  first  will  generally  be  chosen,  at  least  by  those  without 
special  musical  training.     The  tones  should  be  sounded  suc- 
cessively, not  at  the  same  time,  during  the  test.     Afterward 
they  may  be  sounded  together,  and  the  pitch  of  the  bottle 
determined  approximately  by  finding  with  which  tone  of 
the  instrument  its  tone  makes  the  slowest  beats  (cf.  Ex. 
79).     It  should  be  remembered,  however,  that  it  will  be 
possible  to  get  beats  also  with  tones  an  octave  lower  and  an 
octave  higher  than  that  corresponding  most  nearly  with  the 
true  pitch  of  the  bottle  tone. 

Stumpf,  L,  227-247,  especially,  235-245. 


76]  SENSATIONS    OF  HEARING.  65 

75.  Recognition  of  Musical  Intervals.     Cause  a  familiar 
air  to  be  played,  first  in  the  octave  of  c  and  then  in  that  of 
c"  in  the  same  or  another  key.     Even  those  of  no  musical 
training  will  easily  recognize  that  the  air  (i.  e.,  the  succes- 
sion of  musical  intervals  in  fixed  rhythmical  relations),  is 
the  same  in  both  cases ;  and  any  mistake  or  variation  will 
be  noticed  as  easily  as  if  the  air  had  been  repeated  at  the 
first  pitch.     With  the  unmusical,  however,  the  recognition 
is  often  rather  of  the  rhythm  than  the  intervals ;  try  there- 
fore a  repetition  of  the  air  changing  some  of  the  intervals 
but  preserving  the  original  rhythm.     The  power  of  recog- 
nizing intervals  is  very  much  more   highly  developed  in 
persons  of  musical  training,  but  any  one  that  can  whistle 
a  tune  at  one  pitch  and  repeat  it  recognizably  at  another 
undoubtedly  has  the  rudiments  of  interval  recognition. 

For  exact  methods  of  testing  the  accuracy  of  the  power  of  recog- 
nizing intervals,  see  Preyer,  A,  38-64;  and  Schischmanow,  and  the 
references  given  by  them. 

76.  Pitch  Distances.      Beside   the  interval  relations  of 
tones,  and  overshadowed  by  them  in  musicians,  are  certain 
relations  of  separateness  or  distinctness  or  distance  in  pitch, 
which  do  not  depend  on  the  ratios  of  vibration  rates.     Equal 
musical  intervals  (i.e.,  intervals  between   tones  that  have 
vibration  rates  in  a  fixed  ratio  to  each  other,  e.g.,  C  D  and 
c"  d")  do  not  correspond  to  equal  pitch  distances.     Sound 
the  half-tone  interval  c  c-sharp  through  the  range  of  the  in- 
strument, beginning  in  the  bass  and  ascending.     Notice  the 
increasing  distinctness  and  separation  of  the  tones  as  the 
interval  is  taken  higher  and  higher.     For  the  very  highest 
tones  there  is  probably  a  decrease  of  separateness  again. 
The  difference  is  most   striking,  however,  with   intervals 
smaller  than  those  in  common  use,  e.g.,  with  quarter  or 
eighth  tones.     On  the  harmonical  (cf.  notes  on  apparatus) 
strike  in  succession  the  c-sharp  and  d  keys  in  the  four  lower 


66          LABORATORY   COURSE  IX  PSYCHOLOGY,          [?9 

octaves,  beginning  with  the  lowest.  In  this  instrument  the 
osharp  key  is  given  to  another  d,  a  comma,  or  about  one- 
ninth  of  a  tone,  natter  than  the  regular  d  of  the  scale. 

Stumpf,  I.,  247-253;  Lorenz,  and  the  discussion  between  Wundt, 
Stumpf ,  and  Engel ;  Helmholtz,  264-265 ;  Miinsterberg,  C. 

77.  The  Effect  of  a  Given  Tone  in  a  Melody  depends  in 
part  on  the  succession  of  tones  in  which  it  stands.     Cause  a 
simple  air,  in  which  the  same  tone  recurs  in  different  suc- 
cessions of  tones,  to  be  played,  and  notice  the  difference 
in  effect  in  the  different  circumstances,  or  simply  play  the 
ascending  and  descending  scales. 

Mach,  5,  130-131. 

78.  Tones  that  Vary  Irregularly  in  time  and  in  pitch  are 
unpleasant.     Test  with  a  piston  whistle. 

SIMULTANEOUS  TONES. 

79.  Beats.     When  tones  that  are  different  in  pitch  are 
sounded   at   the   same   time,  they  mutually  interfere,  and 
make  the  total  sensation  at  one  instant  more  intense  and 
the  next  instant  less  intense.     This   regular  variation   in 
intensity  is  called  "  beating."     Exs.  71  and  74,  where  beats 
have  been  used  incidentally,  are  a  sufficient  introduction  to 
them. 

a.  The  rapidity  of  beats  depends  on  the  difference  in  the 
vibration  rates  of  the  beating  tones.  Prepare  two  bottle 
whistles  of  the  same  size,  and  blow  both  at  the  same  time. 
Slow  beats  will  probably  be  heard.  If  not,  pour  a  little 
water  into  one  bottle  (thus  raising  the  pitch  of  its  tone),  and 
blow  as  before.  Continue  adding  water,  a  little  at  a  time, 
till  the  beats  lose  themselves  in  the  general  roughness  of 
the  tone.  Blow  the  bottles  separately  now  and  then  to 
observe  the  increasing  difference  in  pitch.  The  same  may 
be  shown  with  a  couple  of  piston  whistles,  if  they  are  first 


80]  SENSATIONS    OF  HEARING.  67 

adjusted  to  unison,  and  then  the  piston  of  one  or  the  other 
is  slowly  pushed  in  or  pulled  out. 

b.  Tones  that  are  a  little  more  or  a  little  less  than  an 
octave  apart  may  give  beats.     Try  with  a  pair  of  octave- 
forks  on  resonance  boxes  or  held  over  resonance  bottles,  one 
of  which  has  been  slightly  lowered  in  pitch  by  weighting 
the   prongs  with  wax  or  a  bit  of  rubber  tubing.     In  this 
case  the  beating-tones  are  the  tone  of  the  lower  fork  and 
the  difference  tone  (see  Ex.  82).     Eepeat  the  experiment  on 
a   reed   instrument.     In  this  case  beats  may  be  heard  be- 
tween the  higher  tone  and  the  first  over-tone  of  the  lower 
(see  Ex.  86). 

c.  The  rate  at  which  the  roughness  of   rapid  beats  dis- 
appears, as  also  the  rate  which  produces  the  greatest  rough- 
ness, differs  with  the  pitch  of  the  beating-tones.     Sound  the 
following   pairs   of  tones  which   have  somewhat  near  the 
same  difference  in  vibration  rates  per  sec.,  namely,  33 ;   and 
observe  that  the  roughness  from  the  beats   decreases  and 
finally  disappears  entirely  at  about  the  fourth  pair ;  V  c", 
cr  dr,  e  g,  c  e,  G  c,  C  G.     The  a!  and  c"  tuning-forks  give  a 
vanish  of  roughness,  representing  a  rate  of  80-88  per  sec. 

Helmholtz,  159-173;  Stumpf.,  II.,  449-497,  especially  461-465 ; 
Mayer,  A  ;  Cross  and  Goodwin. 

80.  Beats  Betray  the  Presence  of  very  Faint  Tones,  both 
because  the  total  stimulus  is  actually  stronger  in  the  phase 
of  increased  intensity,  and  because  intermittent  stimuli  are 
themselves  more  effective  than  continuous  ones. 

a.  Strike  a  pair  of  beating  tuning-forks,  and  hold  one  at 
such  a  distance  from  the  ear  that  it  is  very  faint  or  quite 
inaudible.     Then  bring  the  other  fork  gradually  toward  the 
ear,  and  notice  the  unmistakable  beats. 

b.  Strike  a  tuning-fork  and  hold  it  at  a  distance,  being 
careful  to  have  the  fork  sidewise  or  edgewise,  not  corner- 


68          LABORATORY   COURSE  IN  PSYCHOLOGY-          [81 

ing,  toward  the  ear.  Rotate  the  fork  one  way  and  the 
other  about  its  long  axis,  and  observe  the  greater  distinct- 
ness of  the  tone,  due  in  this  case  simply  to  its  intermit- 
tence. 

81.  Beats  are  in  general  Attributed  to  the  Tone  that 
Receives  Attention ;  in  the  absence  of  other  determining 
causes,  to  the  louder  tone,  to  the  lower  tone,  or  to  the 
whole  mass  of  an  unanalyzed  compound  tone  (see  intro- 
duction to  Ex.  86). 

a.  Set  two  properly  tuned  resonance  bottles  about  a  foot 
apart  on  the  table.     Strike  two  forks  that  beat,  and  hold 
them  over  the  bottles.     While  both  are  about  equally  in- 
tense, it  is  easy,  by  mere  direction  of  the  attention,  to  make 
the  beats  shift  from  one  to  the  other. 

b.  Turn  one  of  the  forks  an  eighth  of  a  turn  about  its 
long  axis,  which  will  weaken  its   tone,  and  observe  that 
the  beats  seem  to  come  from  the  other  fork.     By  turning 
first  one  fork  and  then  the  other,  the  location  of  the  beats 
may  again  be  made  to  shift  at  pleasure.     If  tuning-forks  on 
resonance  boxes  are  at  hand  they  may  be  used,  and  the  tone 
of  one  weakened  by  covering  the  opening  of  the  box  with  a 
bit  of  cardboard. 

c.  Warm  the  c'  fork  in  any  convenient  way  (holding  it 
clasped  in  the  hand  will  do).     This  will  flatten  it  some- 
what.   Strike  it  and  the  c"  fork,  and  press  the  stems  of  both 
on  the  table  at  the  same  time ;  or,  better,  on  the  sounding- 
board  of  the  sonometer.     Observe  that  the  beats  seem  to 
come  from  the  cf  fork  unless  it  is  very  faint. 

d.  Tune  a  string  of  the  sonometer  so  that  its  third  partial 
(or  corresponding  harmonic)  beats  slowly  with  the  c"  fork. 
(On  partials  and   harmonics  cf.  Exs.   86-89.)     Strike   the 
tuning-fork,  and  hold  it  over  a  resonance  bottle,  or  press  its 
stem   against   the   table  at  arm's  length  from   the    string. 
Then  pluck  the  string  and  attend  to  its  tone ;  the  beats  may 


82]  SENSATIONS    OF  HEARING.  69 

seem  to  affect  the  whole  compound  tone  of  the  string. 
But  this  will  not  happen  if  the  tone  of  the  string  is  an- 
alyzed, or  if  the  attention  is  directed  to  the  fork.  The 
same  may  be  tried  on  the  piano  by  picking  out  from  the 
mistimed  c"  forks  one  that  beats  slowly  with  c"  on  the 
piano.  Strike  the  /  key  and  hold  it  down ;  strike  the  fork, 
and  observe  the  beats  as  before.  Cf.  Ex.  69  a. 
Stumpf,  II.,  489-497. 

82.  Combination  Tones:  Difference  Tones.1  When  two 
tones  are  loudly  sounded  at  the  same  time  they  produce  by 
their  combination  other  tones,  one  of  a  pitch  represented  by 
the  difference  of  the  vibration  rates  of  the  two  original  or 
generating  tones,  and  one  of  a  pitch  corresponding  to  their 
sum.  The  existence  of  the  summation  tones  has  been  dis- 
puted, and  they  are  hard  to  hear.  The  difference  tones, 
however,  are  easy  to  hear,  at  least  when  they  are  consider- 
ably lower  -in  pitch  than  the  generators,  when  the  latter  are 
loud  and  sustained,  and  when  they  make  a  consonant  in- 
terval —  though  the  last  is  not  essential.  A  loud  difference 
tone  may  itself  take  the  part  of  a  generator  and  produce  yet 
another  difference,  tone  —  a  difference  tone  of  the  second 
order  —  and  so  on,  though  difference  tones  of  higher  orders 
are  heard  with  difficulty  even  by  skilled  observers.  Differ- 
ence tones  are  hard  to  hear  on  the  piano  and  similar  stringed 
instruments  because  of  the  rapid  decline  in  the  strength  of 
the  generators.  The  difference  tones  are  sometimes  called 
Tartini's  tones,  after  an  early  observer  of  them. 

a.  Repeat  Ex.  79  a,  continuing  to  pour  water  into  one  of 
the  bottles  till  the  difference  tone  appears.  At  first  the 
roughness  of  the  beats  and  the  difference  tone  may  both  be 


1  Konig  distinguishes  between  "  difference  tones  "  find  "  beat  tones."  Both 
tones,  however,  generally  have  the  same  pitch,  and  the  older  term  for  them  has 
here  been  retained  ;  strictly  speaking,  however,  the  "  difference  tones  "  heard  in 
these  experiments  are  "  beat  tones." 


70          LABORATORY   COURSE  IN  PSYCHOLOGY.          [82 

heard  at  once.  Try  the  same  with  the  piston  whistles,  first 
setting  them  at  unison,  and  then  slowly  pushing  the  piston 
of  one  in  or  out  while  blowing  rather  hard.  The  beats  will 
almost  immediately  give  place  to  a  low  difference  tone 
which  may  be  heard  ascending  through  several  octaves 
before  becoming  indistinguishable  from  the  generators. 
The  double  warning  whistles  used  by  bicyclists  give  a  fine 
difference  tone,  to  which  indeed  they  owe  their  deep  and 
locomotive-like  quality. 

b.  Difference  tones  are  strong  on  reed  instruments.     Press 
the  adjacent  white  keys  of  a  parlor  organ,  or  the  harmonical, 
by  twos,  beginning  at  c  and  going  up  a  couple  of  octaves. 
If  there  is  difficulty  in  hearing  the  difference  tone,  sound 
the  upper  tone  intermittently  and  listen  for  the  difference 
tone  at  the  instant  of  pressing  the  key. 

c.  Sound  c"  and  d"  which  should  give  C  as  a  difference 
tone  (594—528=66).     Sound  also  d"  and  e"  which  should 
give  the  same  (660—594=66).     If,  however,  the  tuning  is 
inexact,  as  it  is  intentionally  in  the   tempered  tuning  of 
keyed  instruments,  these  difference  tones  will  be  somewhat 
different  and  may  be  heard  to  beat  with  each  other  when  c", 
d"  and  e"  are  sounded  at  once.     Notice  that  these  beats  are 
not  heard  when  the  tones  are  sounded  in  pairs.     On  the 
harmonical  this  difference  may  be  brought  about  by  sound- 
ing one  of  the  tones  fiat  by  pressing  its  key  only  a  little 
way   down.     The   same   thing   may  be    shown  with   three 
piston  whistles  blown  at  once,  by  a  little  careful  adjustment 
of  the  pistons. 

d.  In  the  case  of   reed  instruments  the  difference  tones 
probably  owe  part  of  their  intensity  to  the  vibrations  of  the 
air  in  the  wind  chest.     When  two  whistles  are  blown  by  one 
person  something  of  the  same  kind  may  happen.     In  order 
to  make  a  clean  experiment,  have   the  whistles  blown  by 
two  assistants,  or  observe  the  difference  tones  from  tuning- 
forks. 


83]  SENSATIONS   OF  HEARING.  71 

e.  The  location  of  difference  tones.  The  location  of  these 
tones  is  sometimes  influenced  by  the  location  of  their 
generators,  but  under  favorable  circumstances  they  seem  to 
arise  in  the  ears  or  even  in  the  head.  This  is  strikingly  the 
case,  both  for  the  blower  and  the  listeners,  with  the  differ- 
ence tones  produced  with  the  piston  whistles.  Cf.  Ex.  69  e. 

Helmholtz,  152-159;  Stumpf,  II.,  243-257;  Konig;  Preyer,  C  and 
Z>;  Hermann. 

83.  Blending  of  Tones.  The  degree  to  which  tones  blend 
with  one  another  differs  with  the  interval  relation  of  the 
tones  taken.  It  is,  according  to  Stumpf,  greatest  with  the 
octave,  less  with  the  fifth,  less  again  with  the  fourth,  slight 
with  the  thirds  and  sixths,  and  least  of  all  with  the  remain- 
ing intervals. 

a.  Try  on  the  instrument  the  extent  to  which  the  tones 
forming  these  intervals  blend,  also  those  forming  intervals 
greater  than  the  octave  :  double  octave,  twelfth,  etc. 

b.  The  blending  in  case  of  the  octave  is  so  complete  under 
favorable  circumstances  as  to  escape  the  analysis  of  trained 
ears.     Use  two  tuning-forks,  one  an  octave  higher  than  the 
other,  on  resonance  cases  or  held   over  resonance  bottles. 
Sound  the  forks,  first  the  higher,  then  the  lower.     For  a 
while  the  higher  fork  will  be  heard  sounding  in  its  proper 
tone,  but  by  degrees  it  will  become  completely  lost  in  the 
lower,  and  a  subject  with  closed  eyes  will  be  unable  to  say 
whether  or  not  it  yet  sounds.    Cf.  Ex.  69  b.    Stop  the  lower 
fork,  or  remove  it  from  its  resonance  bottle,  and  notice  that 
the  higher  is  still  sounding.     Notice  the  change  in  timbre 
(cf .  Ex.  90)  produced  by  the  stopping  of  the  higher  fork  — 
something  like  the  change  from  the  vowel  0  to  the  vowel 
U  (oo). 

On  a,  Stumpf,  II.,  127-218,  especially  135-142;  for  his  experi- 
ments on  the  unmusical  confirming  his  grades  of  blending,  142-173, 
On  6,  Stumpf,  II.,  352-358,  and  Helmholtz,  00-61. 


72          LABORATORY  COURSE  IN  PSYCHOLOGY.          [85 

84.  Analysis  of  Groups  of  Simultaneous  Tones.     Ease  of 
analysis  depends  011  a  number  of  conditions,  among  others 
on  the  following. 

a.  Analysis  is  easier  for  tones  far  distant  in  the  scale. 
Compare  the  ease  of  recognizing  the  sound  of  the  c"  fork 
when  c'  and  c"  are  sounded  together,  with  that  of  recogniz- 
ing c"f  when  sounded  with  c' .     Compare  also  the  ease  of 
distinguishing  c'  and  a!  with  that  of  distinguishing  c'  and 
a". 

b.  Analysis  is  made  easier  by  loudness  in  the  tone  to  be 
separated.     Repeat  Ex.  83  b,  sounding  the  c'  faintly,  the  c" 
strongly.     Little  difficulty  will  be  found  in  keeping  the  lat- 
ter distinct. 

c.  Analysis  is  easier  when  the  tones  make  intervals  with 
little  tendency  to  blend.     Compare  the  ease  of  analysis  of 
c'  c"  and  cr  a!  or  a'  c" .     Also  notice  that  the  addition  of  d" 
(octave  of  d',  fifth  of  gf,  fourth  below  g")  to  the  chord  g  df 
g'  g"  produces  a  less  striking  change  than  the  addition  of  b' 
(major  third  of  cf,  minor  sixth  below  g")  to  the  same  chord. 

d.  Analysis   is   easier  with   sustained   than  with   short 
chords.     Repeat  the  last  experiment,  making  the  chords  very 
short,  and  notice  that  the  difference  made  by  inserting  either 
d"  or  b'  is  less  marked.     Cf.  also  Ex.  100. 

Stumpf,  II.,  318-361;  also  his  experiments,  362-382. 

85.  The   Lower   Tone  of   a   Chord  Fixes   the  Apparent 
Pitch  of  the  Whole,     a.   Repeat  Ex.  83  b,  and  notice  that 
when  the  c'  fork  is  stopped,  the  tone  appears  to  jump  up- 
ward an  octave  in  pitch  (i.e.,  it  takes  the  pitch  of  the  G"  still 
sounding)  ;  but  when  the  c"  fork  is  removed,  the  quality  of 
the  tone  is  changed,  but  not  its  pitch. 

b.  Strike  the  chord  C  c"  e"  g"  or  G  e'  g'  c" ,  and  compare 
the  effect  upon  the  pitch  of  the  whole  mass  of  tone  pro- 
duced by  omitting  C  or  G  alone  with  that  of  omitting  any 
one  or  all  three  of  the  higher  tones.  See  also  the  function 


86]  SENSATIONS    OF  HEARING.  73 

of  the  lowest   partial  of  a  compound  tone  in  fixing  the 
pitch,  noticed  below. 
Stumpf,  II.,  383-392. 

86.  Compound  Tones.  Almost  all  tones  heard,  and  in- 
deed all  those  used  in  music,  are  not  simple  tones,  but  com- 
pound. The  tone  given  by  the  C  string  of  a  piano  is  made 
up  of  at  least  C,  c,  g,  cf,  e'  and  g',  and  generally  other  tones. 
The  lowest  tone  of  the  group  gives  the  pitch  attributed  to 
the  whole,  and  is  known  as  the  fundamental,  the  other  tones 
as  over-tones.  In  another  way  of  naming  them,  the  com- 
ponent tones  are  all  partial  tones  or  partials,  the  fundamen- 
tal being  called  the  first  or  prime  partial,  the  next  higher 
the  second  partial  and  so  on.  The  first  over-tone  is  thus 
the  second  partial  tone,  the  second  over-tone  the  third  partial, 
and  in  general  the  same  tone  receives  as  a  partial  tone  a 
number  one  higher  than  as  an  over-tone.  The  vibration 
rates  of  the  partial  tones  of  a  compound  are  generally  once, 
twice,  three  times,  four  times,  the  rate  of  the  fundamental, 
and  so  on.  In  some  cases,  however,  e.g.,  in  bells  and  tun- 
ing-forks, one  or  more  of  the  partial  tones  may  have  a  vibra- 
tion rate  not  represented  in  this  series,  and  discordant 
with  the  fundamental  tone.  In  what  follows,  the  regular 
series  of  partial  tones  is  meant  except  where  the  contrary 
is  specified. 

Partial  Tones.  If  resonators  are  at  hand,  the  demon- 
stration of  the  partial  tones  will  be  easy.  Sound  on  a 
stringed  or  reed  instrument  the  tones  to  which  the  resona- 
tors are  tuned,  and  notice  that  they  resound  strongly  to 
these  tones  and  less  strongly  or  not  at  all  to  other  tones 
adjacent  in  pitch.  Then  sound  the  tone  to  which  the  lar- 
gest of  the  resonators  is  tuned  (or  a  tone  an  octave  lower), 
and  try  the  resonators  in  succession.  Notice  that  others 
also  resound  (at  their  own  proper  pitch),  thus  betraying 
the  presence  of  the  tones  to  which  they  are  tuned,  and 


74          LABORATORY   COURSE  IN  PSYCHOLOGY.         [88 

thus  the  composite  character  of  the  tone  under  examination. 
Which  resonators  will  "  speak  "  will  depend  on  the  instru- 
ment used  ;  reed  instruments  give  a  long  and  perfect  series, 
piano  and  stretched  wires  a  perfect  series  generally  as  far 
as  the  ninth  or  tenth  partial,  and  stopped  organ-pipes  a 
short  series.  If  difficulty  is  found  in  knowing  when  the 
resonator  is  resounding,  it  will  be  found  useful  to  apply  it 
to  the  ear  intermittently,  alternating,  for  example,  two  sec- 
onds of  application  with  two  seconds  of  withdrawal. 

87.  Partial  Tones  :    Analysis  by  indirect  means,     a.   By 
sympathetic  vibration.     This  succeeds  especially  well  with 
the  piano.     Press  the  c  key  and  hold  it  down  so  as  to  leave 
its  strings  free  to  vibrate  ;  then  strike  the  C  key  forcibly, 
and  after  one  or  two  seconds  release  it.    The  c  strings  will  be 
found  to  be  sounding.     Repeat,  trying  c-sharp  or  b  instead 
of  e;  they  will  be  found  not  to  respond.     Kepeat  the  experi- 
ment, substituting  g,  cf,  er,  g',  or  c" ;  all  will  be  found  to  re- 
spond but  in  lessening  degrees.     Other  keys  between  C  and 
c"  may  be  tried  but  will  be  found  in  very  faint  vibration,  if 
at  all. 

b.  By  beats.  This  will  succeed  best  with  a  reed  instru- 
ment, e.  g.,  a  parlor  organ  or  the  harmonical.  By  pressing 
the  keys  of  the  instrument  only  a  little  way  down,  any  of  its 
tones  may  be  sounded  a  little  natter  than  its  true  pitch  and 
so  in  condition  to  beat  with  any  other  tone  having  that  true 
pitch.  Sound  at  this  flattened  pitch  the  over-tones  of  C  in 
succession  while  C  is  sounding,  and  notice  the  slow  beats 
that  result.  For  verification  sound  other  tones  not  over-tones 
of  (7,  and  notice  that  the  beats  when  present  are  much  more 
rapid. 

88.  Partial  Tones  :  Direct  analysis  without  special  appara- 
tus.    The  directions  given  here  apply  to  the  sonometer,  but 
will  be  readily  adaptable  to  any  stringed  instrument  in  which 


88]  SENSATIONS   OF  HEARING.  75 

the  strings  can  be  exposed.  It  is  easier  to  hear  any  partial 
tone  in  the  compound,  if  the  partial  is  first  heard  by  itself, 
and  then  immediately  in  combination  with  the  rest.  On 
strings  this  is  easily  done  by  sounding  the  partials  as 
"  harmonics."  Pluck  the  string  near  one  end  (say  about 
one-seventh  of  the  length  of  the  string  from  the  end),  and 
immediately  touch  it  in  the  middle  with  the  finger  or  a 
camel's-hair  brush.  The  fundamental  will  cease  to  sound 
and  its  octave  (the  second  partial)  will  be  left  sounding,  as 
a  "  harmonic."  With  it  sound  also  other  even-numbered 
partials,  but  less  strongly.  Pluck  as  before,  and  touch  the 
string  at  one-third  its  length ;  the  third  partial  will  now 
sound  out  strongest,  with  the  sixth,  ninth,  etc.,  more  faintly. 
Thus  by  plucking  the  string  and  touching  it  respectively 
at  one  half,  one  third,  one  fourth,  one  fifth,  one  sixth,  one 
seventh,  one  eighth,  one  ninth,  and  one  tenth  its  length  from 
the  end,  the  series  of  tones  corresponding  to  the  2d,  3d,  4th, 
5th,  6th,  7th,  8th,  9th,  and  10th  partials  can  be  heard,  each 
in  large  measure  by  itself.  In  getting  the  higher  "harmon- 
ics "  it  will  be  found  better  to  pluck  nearer  the  end  than 
one  seventh,  and  in  no  case  should  the  string  be  plucked  at 
the  point  at  which  it  is  presently  to  be  touched.  (Of.  Ex. 
906.) 

To  hear  the  partial  tones  when  sounding  in  the  compound, 
proceed  as  follows.  Sound  the  required  tone  as  a  "har- 
monic," and  then  keeping  the  attention  fixed  on  that  tone, 
stop  the  string  and  pluck  it  again,  this  time  letting  it  vibrate 
freely.  The  tone  just  heard  as  a  "  harmonic  "  will  now  be 
heard  sounding  with  the  rest  as  a  partial.  When  the  partial 
is  thus  made  out,  verify  the  analysis  by  touching  the  string 
again  and  letting  the  tone  sound  once  more  as  a  "harmonic." 
Try  in  this  way  for  the  partials  up  to  the  tenth ;  first  for 
the  3d,  5th,  and  7th,  afterward  for  the  6th,  4th,  and  the  2d, 
which  is  the  most  difficult  of  all.  It  is  said  that  analysis 


78          LABORATORY  COURSE  IN  PSYCHOLOGY.          [90 

is  easier  at  night  (not  alone  011  account  of  the  greater  still- 
ness) and  when  one  ear  is  used,  and  that  certain  positions 
of  the  head  favor  certain  partials. 

89.  Partial   Tones :   Direct   analysis  without  apparatus. 
Certain  parts  of  a  compound  tone  are  sometimes  so  sepa- 
rated by  their  dissonance,  intensity,  or  pitch  that  they  stand 
out  with  striking  clearness. 

a.  Strike  a  tuning-fork  on  a  hard  surface,  and  observe  the 
high,  ringing,  dissonant  partials.     They  fade  out  before  the 
proper  tone  of  the  fork,  and  are  heard  best  when  the  fork 
is  not  held  near  the  ear. 

b.  As  the  tone  of  a  string  is  allowed  to  die  away  of  itself, 
different  partial  tones  come  successively  into  prominence. 
Try  with  a  low  piano  string,  keeping  the  key  pressed  down 
while  the  sound  fades,  or  with  the  sonometer.     Something 
of  the  same  kind,  but  less  marked,  happens  in  the  dying 
away  of  a  low  tone  on  a  reed  instrument  when  the  air  is 
allowed  to  run  low  in  the  bellows. 

c.  When  a  tone  is  sounded  continuously  for  some  time  on 
a  reed  instrument  with  one  of  the  keys  clamped  down,  dif- 
ferent partials   come  successively  into  prominence,  either 
through  varying  fatigue  or  the  wandering  of  attention. 

Helmholtz,  36-65;  Stumpf,  II.,  231-243;  see  also  the  index  under 
Obertone ;  Mach,  A,  58,  J5,  127. 

90.  Timbre.     The  peculiar  differences  in  quality  of  tones 
(distinct   from   pitch  and   intensity)  which  are    known  as 
differences  in  timbre  (tone-color,  clang-tint,  Klangfarbe),  are 
due  largely  to  differences  in  the  number,  pitch,  and  intensity 
of  the  partial  tones  present.     Compare  in  this  respect  the 
dull-sounding  bottle-tones  or  the  tones  of  tuning-forks  held 
over  resonance  bottles,  and  the  more  brilliant  tones  of  a  reed 
or  stringed  instrument ;  the  first  are  nearly  simple  tones, 
while  the  second  have  strong  and  numerous  over-tones. 


91]  SENSATIONS    OF  HEARING.  77 

a.  Notice  the  difference  in  quality  between  the  tone  given 
by  a  tuning-fork  held  before  the  ear  and  that  given  by  the 
same  fork  when  its  stem  is  pressed  upon  the  table.     In  the 
second  position  the  over-tones  are  relatively  stronger. 

b.  Notice   the   differences  in   quality  in   the   tone  of  a 
string  when  it  is  plucked  in  the  middle,  at  one  third  its 
length  and  at  about  one  seventh.     When  plucked  in  the 
middle,  many  odd-numbered  partials  are  present,  and  the 
even-numbered  partials  are  either  absent  or  extremely  faint, 
and  the  tone  is  hollow  and  nasal ;  when  plucked  at  one  third, 
the  third,  sixth,  and  ninth  partials  are  wanting,  and  the  tone 
is  hollow,  but  not  so  much  so  as  before ;  when  plucked  at 
one  seventh  all  the  partials  up  to  the  seventh  are  present. 
For  their  theoretical  intensities,  cf.  Helmholtz,  79. 

c.  Try  also  plucking  very  near  one  end,  plucking  with 
the  finger-nail  and  striking  the  string  with  a  hard  body,  e.  g., 
the  back  of  a  knife-blade  ;  all  these  bring  out  the  higher  and 
mutually  discordant  partials  strongly,  and  produce  a  brassy 
timbre. 

Helmholtz,  65-119  ;  Stumpf,  II.,  514-549. 

91.  In  Successive  Chords  the  AVhole  Mass  of  Tone  seems 
to  move  in  the  same  direction  as  the  part  that  changes  most. 
Strike  in  succession  the  chords  e'  gf -sharp  b'  <?",  a  a'  c" -sharp 
e",  or  a  c'  e'  c",  a  cr  f  c".  If  the  attention  is  directed  to  the 
bass  in  the  first  example  and  to  the  alto  in  the  second  the 
whole  mass  of  tone  will  appear  to  descend  in  the  first  case 
and  to  ascend  in  the  second.  If  the  attention  is  kept  on  the 
soprano  part  the  illusion  will  not  appear,  as  also  when  the 
observer  examines  his  sensations  critically.  Cf.  also  Ex.  81 
d,  where  beats  of  a  partial  tone  are  attributed  to  the  whole 
compound  tone. 

Mach,  B,  126-127;  Stumpf,  II.,  393-395. 


78         LABORATORY  COURSE  IX  PSYCHOLOGY.          [93 

92.    Simultaneous    Tones   interfere   somewhat  with   one 
another  in  Intensity. 


a.  Play  the  groups  of  notes  numbered  1,  2,  and  3  and  ob- 
serve the  slight  increase  in  the  apparent  intensity  of  the 
remaining  tones  as  one  after  another  drops  out,  making  1 
sound  like  la,  2  like  2a,  and  so  on.  On  the  piano  it  will  be 
well  to  play  the  notes  an  octave  or  two  lower  than  they 
are  written. 


b.  Play  the  notes  marked  4,  and  notice  that  the  increase 
of  loudness  seems  to  affect  the  note  (highest  or  lowest)  that 
receives  particular  attention,  making  the  effect  in  one  case 
like  4a,  in  the  other  like  46. 

Mach,  B,  126;  Stumpf,  II.,  418-423. 

93.  Consonant  and  Dissonant  Intervals,  a.  The  conso- 
nant intervals  within  the  octave  are  the  unison,  octave,  fifth, 
fourth,  major  sixth,  major  third,  minor  third,  and  minor 
sixth.  They  will  be  found  to  decrease  in  smoothness  about 
in  the  order  given.  Try  them  beginning  with  the  octave 
and  at  c,  as  follows  :  c  c',  eg,  cf,c  a,  c  e,  c  e-ftat,  c  a-flat. 
Try  the  last  four  intervals  also  in  the  octave  of  c"  or  c'" 
and  notice  that  they  are  less  rough  than  when  taken  in  the 


95]  SENSATIONS   OF  HEARING.  79 

octave  of  c.  Any  other  intervals  within  the  octave  are  dis- 
sonant. Try  c  c-sharp,  c  d,  c  b,  c  b-flat,  c  f-sharp.  The 
roughness  is  due  to  beating  partial  tones  and  in  general 
is  greater  when  these  stand  low  in  the  partial  tone  series 
and  are  loud,  and  when  they  lie  within  a  half-tone  of  each 
other.  Work  out  for  the  tones  of  several  of  the  intervals 
the  series  of  partial  tones  up  to  the  eighth.  In  general  the 
extension  of  intervals  into  the  second  octave  (taking  the 
higher  tone  an  octave  higher  or  the  lower  tone  an  octave 
lower)  does  not  change  the  fact  of  consonance  or  dissonance, 
though  it  may  change  the  relative  roughness. 

b.  Those  fitted  by  musical  training  to  pronounce  upon 
questions  of  consonance  and  dissonance  hold  that  dissonance 
can  be  perceived  between  simple  tones  under  conditions  that 
exclude  beats,  and  that  consonance  is  something  more  than 
the  smooth  flowing  of  tones  undisturbed  by  beats.  The 
test  is  easy  to  make.  Hold  tuning-forks  making  the  inter- 
val to  be  tested  one  before  each  ear,  and  if  there  are 
beats,  carry  the  forks  far  enough  away  in  each  direction 
to  make  the  beats  inaudible.  Only  those  of  musical  ear, 
however,  can  pronounce  upon  the  result. 

Helmholtz,  179-197;  Stumpf,  II.,  470,  460;  Wundt,  3te  Aufl.,  I., 
439,  II.,  47  ff;  Mach,  B,  129-130;  Preyer,  D,  44  ff. 

94.  Consonant  and  Dissonant  Chords.     In  order  to  form 
a  consonant  chord,  all  the  intervals  among  the  tones  must 
also  be  consonant.     The  only  chords  of  three  tones  which 
fulfil  this  condition  within  the  octave  are  represented  by 
the  following  :  Major  c  e  g,  cf  a,  e  e-flat  a-flat,  minor  c  e- 
flat  g,  c  f  a-flat,  c  e  a.     Try  these  and  for  comparison  any 
other  chord  of  three  tones  having  c  for  its  lowest  tone. 

Helmholtz,  211  ff.;  Wundt,  3te  Aufl.,  II.,  61,  63  ff. 

95.  Major  and  Minor  Chords.     Compare  the  chords  c"  e" 
g"  and  c"  e"-flat  g" .     This  unmistakable  difference  in  effect 


80          LABORATORY  COURSE  IN  PSYCHOLOGY.          [97 

depends  in  part  at  least  on  the  fact  that  in  the  major  chord 
the  difference  tones  of  the  first  order  are  lower  octaves  of  c" 
itself,  while  in  the  minor  chord  one  difference  tone  is  not 
such  at  all,  and  if  taken  in  the  same  octave  with  the  chord 
would  be  highly  dissonant.  For  the  major  chord,  when 
taken  in  the  octave  of  c",  the  difference  tones  are  c  and  c", 
for  the  minor  chord  c  e-flat,  A-flat.  Try  on  a  reed  instru- 
ment the  difference  tones  generated  by  c"  e",  e"  g",  c"  e"-flat, 
e"-flat  g",  first  separately  ;  and  then,  while  c"  and  g"  are 
kept  sounding  strike  e"  and  e"-flat  alternately. 

Helmholtz,  215-217;  Stumpf,  II.,  335,  376  ff.;  Wundt,  3te.  Aufl., 
II.,  61 ff.,  67  ff. 


96.  Cadences.    Modern  music  requires  the  prominence  of 
the  key  note  or  tonic  and  of  the  chord  in  which  it  holds  the 
chief  place  at  the  beginning  of  a  piece  of  music  and  at  the 
end.     The  feeling  of  the  appropriateness  of  this  close,  and 
especially  of  the  succession  of  chords  in  the  cadences  above, 
can  hardly  fail  to  appeal  even  to  the  unmusical. 

Helmholtz,  293. 

97.  The  Absolute  Time  Relations  of  music  have  much  to 
do  with  its  emotional  effect.     Have  a  familiar  piece  of  music 
played  in  its  proper  time,  then  very  slowly  and  very  rapidly. 


98]  SENSATIONS   OF  HEARING.  81 


I>INAURAL  AUDITION  AND  THE  LOCATION  OF  SOUNDS. 

98.  Unison  Tones  Heard  with  the  Two  Ears.  a.  Strike 
a  pair  of  unison  forks  that  will  sound  equally  loud  and 
vibrate  an.  equal  length  of  time,  and  hold  one  before  each 
ear,  three  or  four  inches  away ;  a  single  tone  of  rather  in- 
definite location  will  be  heard.  As  the  forks  are  brought 
nearer,  their  tone  seems  to  draw  by  degrees  toward  the 
median  plane ;  and  when  they  are  very  loud  and  near,  the 
tone  may  seem  to  be  in  the  head.  Return  the  forks  to 
their  first  position  and  then  move  one  a  little  nearer  or  a 
little  farther  away,  and  notice  that  the  sound  moves  to  the 
side  of  the  nearer  fork.  When  the  difference  in  distance 
has  become  considerable  that  fork  alone  will  be  heard. 

b.  Bring  the  forks  again  into  the  positions  last  mentioned 
—  one  near  and  one  far,  (or  better,  place  one  fork  on  a  rub- 
ber tube  one  end  of  which  has  been  inserted  in  the  opening 
of  the  ear  and  hold  the  other  fork  before  the  other  ear), 
and  then  with  the  free  or  more  distant  fork  make    slow 
rhythmical  motions  toward  and  away  from  the  ear,  or  rotate 
the  fork  slowly  about  its  long  axis,  attending  meantime  to 
the  fork  on  the  other  side.     Alternate   variations  in  the 
intensity  of  the  tone  of  this  fork  corresponding  to  the  ap- 
proach and  recession  of  the  other  and  apparently  unheard 
fork  can  be  observed. 

c.  Repeat  b  and  notice  that  when  the  changes  in  intensity 
are  considerable  there  is  a  simultaneous  shifting  of  the  place 
of  the  tone,  towards  the  median  plane  when  the  tone  grows 
stronger,  and  away  when  it  grows  fainter.     These  changes  of 
place  are,  however,  less  marked  than  the  changes  in  intensity 
and  those  accompanying  slight  changes  in  intensity  gener- 
ally escape  observation. 

Schaefer,  B',  Thompson;  Urbantschitseh,  B. 


82         LABORATORY  COURSE  IN  PSYCHOLOGY.       [101 

99.  Beats  Heard  with  Two  Ears.     a.   Operate  as  in  Ex. 
98  a,  with  forks  beating  three  or  four  times  a  second. 

b.  Try  with  a  pair  of  very  slow  beating  forks  (once  in 
two  or  three  seconds).     Notice  a  shifting  of  the  sound  from 
ear  to  ear  corresponding  to  the  rate  of  beating. 

c.  Try  again  with  a  pair  of  rapid  beating  forks  (twenty  or 
thirty  a  second),  and  notice  that  the  beats  are  heard  in  both 
ears. 

Schaefer,  A,  J5,  and  C;  Thompson;  Cross  and  Goodwin. 

100.  Difference  of  Location   Helps   in  the  Analysis  of 
Simultaneous  Tones.     Compare   the   ease  with  which   the 
tones  of  a  pair  of  octave  forks  are  distinguished  when  the 
forks  are  held  on  opposite  sides  of  the  head  with  the  diffi- 
culty of  analysis  in  Ex.  83  b. 

Stumpf,  II.,  336,  363. 

101.  Judgments   of  the   Direction   of   Sounds.      These 
depend  in  general  on  the  relative  intensity  of  the  sounds 
reaching  the  two  ears,  but  there  is  pretty  good  reason  to 
believe  that  other  factors  co-operate  and  that  tolerably  cor- 
rect judgments,  both  as  to  distance  and  direction,  can  some- 
times be  made  from  the  sensations  of  one  ear. 

a.  Let  the  subject  be  seated  with  closed  eyes.     Snap  the 
telegraph  snapper  at  different  points  in  space  a  foot  or  two 
distant  from  his  head,  being  very  careful  not  to  betray  the 
place  in  any  way,  and  require  him  to  indicate  the  direc- 
tion of  the  sound.     Try  points  both  in  and  out  of  the  median 
plane.     Observe  that  the  subject  seldom  or  never  confuses 
right  and  left  but  often  makes  gross  errors  in  other  direc- 
tions.    Constant  tendencies  to  certain  locations  are  by  no 
means  uncommon. 

b.  Have  the  subject  hold  his  hands  against  the  sides  of 
his.  head  like  another  pair  of  ears,  hollow  backward,  and 
try  the  effect  upon   his  judgment  of  the  direction  of  the 
snapper. 


102]  SENSATIONS   OF  HEARING.  83 

c.  Find   approximately   how   far   the   snapper   must  be 
moved  vertically  from  the  following  points  in  order  to  make 
a  just  observable  change  in  location :  on  a  level  with  the  ears 
in  the  median  plane  two  feet  in  front ;   opposite  one  ear, 
same  distance ;  in  the  median  plane  behind  the  head,  same 
distance.     Find  the   just   observable   horizontal   displace- 
ments at  the  same  points.     A  convenient  way  of  measuring 
these  distances  is  to  clamp  a  yard-stick  to  a  retort-stand, 
bring  it  into  the  line  along  which  measurements  are  to  be 
made  and  hold  the  snapper  over  the  divisions  of  the  stick. 
Snap  once  at  the  point  of  departure,  then  at  a  point  a  little 
way  distant  in  the  direction  to  be  studied ;  again  at  the 
first  point,  so  that  the  subject  may  keep  it  in  mind,  and 
then  at  a  point  a  little  more  distant,  and  so  on  till  a  point 
is  finally  found  which  the  subject  recognizes  as  just  obser- 
vably different.     Repeat,  alternating  snaps  at  the  point  of 
departure  with  those  at  a  greater  distance  than  that  just 
found,  decreasing  the  latter  till  a  point  is  found  where  the 
directions  can  be  no  longer  distinguished.     Make  a  number 
of  tests  each  way  and  take  their  average. 

d.  Continuous  simple  tones  are  very  difficult  to  locate. 
Place  a  tuning-fork  on  its  resonance  case  at  some  distance 
in  front  of  the  subject  (seated  with  closed  eyes),  another  at 
an  equal  distance  behind  him.     With  the  help  of  an  assis- 
tant strike  both  forks,  and  after  a  little  have  one  of  them 
stopped  and  the  mouth  of  its  resonance  box  covered.     Re- 
quire  the   subject  to  say  which  has   been   stopped.     His 
errors  will  be  very  frequent.     Compare  with  this  his  ability 
to  distinguish  whether  a  speaker  is  before  or  behind  him. 

On  a,  Preyer,  B ;  von  Kries,  A ;  on  c,  Miinsterberg,  B ;  on  d,  Ray- 
leigh. 

102.  Intercranial  Location  of  Sounds,  a.  Sounds  origi- 
nating outside  the  head  are  not  located  in  the  head  when 
heard  with  one  ear.  Hold  a  loud-sounding  tuning-fork 


84         LABORATORY  COURSE  IN  PSYCHOLOGY.        [103 

near  the  ear,  or  place  it  on  a  rubber  tube,  one  end  of  which 
is  inserted  in  the  opening  of  the  ear,  and  notice  that  the 
sound  when  strong  may  be  located  in  the  ear,  but  does  not 
penetrate  farther.  Insert  the  other  end  of  the  tube  in  the 
opening  of  the  other  ear  and  repeat.  The  tone,  if  loud,  will 
appear  to  come  from  the  inside  of  the  head.  Removing  and 
replacing  the  fork  several  times  will  help  to  give  definite- 
ness  to  the  location. 

b.  Repeat  the  experiment,  but  use  a  fork  sounding  as 
faintly  as  possible  (e.g.,  set  in  vibration  by  blowing  smartly 
against  it),  and  notice  that  the  location,  when  a  single  ear 
receives  the  sound,  is  not  so  clearly  in  the  ear,  and,  when 
both  receive  it,  not  so  clearly  in  the .  head,  perhaps  even 
outside  of  it.  Cf.  also  Ex.  103  b.  Both  a  and  b  may  also 
be  made  with  beating  tones  instead  of  a  single  one.  See 
also  Ex.  69  e. 

Schaefer,  B. 

103.  Location  of  the  Tones  of  Tuning-forks  Pressed 
against  the  Head.  a.  Strike  a  large  and  loud-sounding 
tuning-fork,  and  press  its  stem  against  the  vertex.  The 
tone  will  seem  to  come  from  the  interior  of  the  head,  chiefly 
from  the  back.  While  the  fork  is  in  the  same  position, 
close  one  of  the  ears  with  the  finger,  not  pressing  it  too 
tight;  the  sound  will  immediately  seem  to  concentrate  in 
the  closed  ear.  Have  an  assistant  manage  the  fork,  and 
close  the  ears  alternately.  Something  of  the  same  kind 
happens  when  a  deep  note  is  sung ;  close  first  one  ear  and 
then  both,  and  notice  the  passage  of  the  tone  from  the 
throat  to  the  ear  and  finally  to  the  middle  of  the  head. 

b.  Have  an  assistant  manage  the  fork,  and  close  both  ears. 
Notice  that  when  the  fork  is  pressed  on  so  as  to  make  the 
tone  loud  the  intercranial  location  is  exact,  but  when  the 
pressure  is  relaxed  and  the  tone  is  faint  the  location  tends 
to  be  extracranial. 


103]  SENSATIONS   OF  HEARING.  85 

c.  Try  setting  the  fork  on  other  places  than  the  vertex. 
Notice  that  in  the  occipital  and  parietal  regions  the  sound 
appears  in  the  opposite  ear,  though  closing  the  ear  as  in 
a  may  bring  it  back  to  the  same  side  as  the  fork. 

d.  Take  a  long  pencil  in  the  teeth  like  a  bit  and  rest  the 
stem  of   a  vibrating  tuning-fork  vertically  on  it  near  one 
end  and  close  the  ear  on  the  other  side ;  the  sound  will 
seem  to  be  located  in  the  closed  ear.     Then  gradually  tilt 
the  fork  backward  toward  a  horizontal  position,  keeping  it 
in  contact  with  the  pencil,  till  its  tip  is  opposite  the  open 
ear.     The  tone  will  change  its  place  from  the  closed  to  the 
open  ear. 

On  a  and  6,  Schaefer,  B  and  C ;  on  c,  Thompson. 


BIBLIOGRAPHY. 

BKUCKE:  Ueber  die  Wahrnehmung  der  Gerausche,  Wien.,  Sitzb.  Sie 

Abth.,  XC.,  1884,  199-230. 
YON  BEZOLD:  A.  Schuluntersuchungen  liber  das  kindliche  Gehoror- 

gan,  Zeitsch.f.  Ohrenheilkvnde,  XIV.,  1884-85,  and  XV.,  1885- 

86;  also  in  English  translation  in  the  Archives  of  Otology,  XIV. 

This  paper  gives    the  results  of   numerous   tests  on  Munich 

school-children,  not  only  with  the  watch  but  also  with  the  acou- 

meter  of  Politzer  and  with  whispered  speech. 
B.  Einige  weitere  Mitteilungen  iiber  die  kontinuierliche  Tonreihe, 

insbesondere   liber  die  physiologische  obere   und  untere  Ton- 

grenze,  ibid.,  XXIII.,  1892,  254-267;  also  in  English  translation, 

Archives  of  Otology,  1893,  216-225. 
COKRADI:   Zur  Priifung  der  Schallperception   durch  die  Knochen, 

Archiv  fur  Ohrenheilkunde,    XXX.,   1890,   175-182.      Review 

with  extract  in  the  Zeitscfirift  fur  Psychologie,  II.,  1891,  124. 
CHARPENTIER:    Recherches   sur  1'intensite   comparative   des   sons 

d'apres  leur  tonalite,  Archives  de  physiologic,  normale  et  patho- 

logique,  1890,  No.  3,  496-507. 


86  LABORATORY   COURSE  IN  PSYCHOLOGY. 

CROSS  AND  GOODWIN:  Some  Considerations  regarding  Helmholtz's 
Theory  of  Consonance,  Proceedings  of  the  American  Academy 
of  Arts  and  Sciences,  1891-92,  1-12. 

CROSS  AND  MALTBY  :  On  the  Least  Number  of  Vibrations  Neces- 
sary to  Determine  Pitch,  ibid.,  222-235. 

DOCQ:   Rechercb.es  physico-physiologique  sur  la  fonction  collective 
des  deux  organs  de  Fappareil  auditif.     Memoir es  couronnes  de 
rAcademie  royale  de  Belgique,  XXXIV.,  1870. 

EXNER:  Zur  Lehre  von  den  Gehorsempfindungen,  Pfluger's  Archiv, 
XIII.,  1876,  228-253. 

HELMHOLTZ  :  Sensations  of  Tone,  English  translation  by  Ellis,  2d 
Ed.,  London,  1885.  This  is  the  great  classic  of  the  subject. 

HENSEN:  Physiologic  des  Gehors,  Hermann's  Handbuch  der  Physio- 
logic, III.,  pt.  2,  1-137. 

HERMANN:  Zur  Theorie  der  Combinationstone,  PJiuger^s  ArcJiiv, 
XLIX.,  1891,  499-518. 

HERROUN  AND  YEO:  Note  on  the  Audibility  of  single  Sound 
Waves  and  the  Number  of  Vibrations  necessary  to  produce  a 
Tone,  Proc.  Royal  Soc.,  L.,  No.  305,  1892,  318-323. 

JAMES  :  Principles  of  Psychology,  New  York,  1890. 

KESSEL:   Ueber  die  vordere  Tenotomie,  Archiv  fur  Ohrenheilkunde, 

XXXI.,  1891,  131-143,  Reviewed,  Zeitschrift  fur  Psychologic, 

II.,  1891,  398. 

KONIO:   Quelques  experiences  d'acoustique,  Paris,  1882. 
VON  KRIES:  A.  Ueber  das  Erkennen  der  Schallrichtung,  Zeitschrift 

fur  Psychologic,  L,  1890,  235-251,  488. 
B.  Ueber  das  absolute  Gehor,  Ibid.,  III.,  1892,  257-279. 
L ANGE  :   Beitrage  zur  Theorie  der  sinnlichen  Auf merksamkeit  und 

der   activen   Apperception,    Wundfs   Philosophische   Studien, 

IV.,  1888,  390-422. 
LORENZ:   Untersuchungen  iiber  die  Auffassung  von  Tondistanzen, 

WundV  s  Philos.  Studien,  VI.,  1890,  26-103. 
LUFT  :     Ueber    die    Unterschiedsempfindlichkeit    fiir    Tonhohen. 

Wundf s  Philos.  Studien,  IV.,  1888,  511-540. 
MACH:  Works  cited  with  same  letters  in  bibliography  of  Chap.  II. 


SENSATIONS   OF  HEARING.  87 

MAYEK:  A.  Researches  in  Acoustics,  Amer.  Jour.  Science,  3d  Ser. 

VIII.,  1874,  241-255,  IX.,  1875,  267-269,  also  Phil.  Mag.,  4th 

Ser.  XLIX.,  Jan.-June,  1875,  352. 
B.   Researches  in  Acoustics,  No.  VIII.,  Amer.  Jour.  Sc.,  3d  Ser. 

XII.,  1876,  329-336,  also  Phil.  Mag.,  Ser.  5,  II.,  July-Dec., 

1876,  500-507. 
MUNSTERBERG:   A.  Schwankungen  der  Aufmerksamkeit,  Beitroge 

zur  experimentellen  Psychologic,  Heft  2,  1889,  69-124. 

B.  Raumsinn  des  Ohres,  Ibid.,  182-234. 

C.  Vergleichung  von  Tondistanzen,  Ibid.,  Heft  4,  1892,  147-177. 
PREYER:   A.  Ueber  die  Grenzen  der  Tonwahrnehmung,  Sammlung 

physiologischer  Abhandlungen,  I.,  Jena,  1877,  1-72. 

B.  Die   Wahrnehmung   der   Schallrichtung   mittelst   der   Bogen- 
gange,  Pfliiger's  Archiv,  XL.,  1887,  586-622. 

C.  Ueber  Combinationstone,    Wledemann's  Annalen,  XXXVIII., 
1889,  131-136. 

D.  Akustische  Untersuchungen,  Sammlung  physiologischer  Ab- 
handlungen, II.,  Jena,  1882,  175-244. 

RAYLEIGH:  Our  Perception  of  the  Direction  of  a  Source  of  Sound, 
Nature,  XIV.,  1876,  32.  See  also  Acoustical  Observations, 
Phil.  Mag.,  Ser.  5,  III.,  Jan.-June,  1877,  456-458. 

RUTHERFORD  :  A  Lecture  on  the  Sense  of  Hearing,  delivered  before 
the  British  Association  at  Birmingham  on  Sept.  6,  1886, 
Lancet,  1887,  i.  2-6. 

SCHAEFER:  A.  Ueber  die  Wahrnehmung  und  Lokalisation  von 
Schwebungen  und  Differenztonen,  Zeitschrift  fur  Psychologie, 
I.,  1890,  81-98. 

B.  Zur  interaurealen    Lokalisation   diotischer  Wahrnehmungen, 
Ibid.,  I.,  1890,  300-309. 

C.  Ein  Versuch  iiber  die  intrakranielle  Leitung  leisester  Tone  von 
Ohr  zu  Ohr,  Ibid.,  II.,  1891,  111-114.     See  also  discussion  of 
Schaefer,  Scripture  and  Wundt,  Ibid.,  IV.,  348;  V.,  397;  and 
Wundfs  Philos.  Studien,  VII.,  630;  VIII.,  638,  641. 

SCHISCHMANOW:  Untersuchungen  iiber  die  Empfindlichkeit  des  In- 
tervallsinnes,  Wundt' s  Philos.  Studien,  V.,  1889,  558-600. 

STUMPF:  Tonpsychologie,  Leipzig,  1883  and  1890.  This  work  of 
Stumpf  s  is  by  far  the  most  complete  upon  the  Psychology  of 
Tone.  The  two  volumes  so  far  published  (the  work  is  to  be 
complete  in  four)  cover  the  psychology  of  successive  and  of 
simultaneous  tones. 


88  LABORATORY  COURSE  IN  PSYCHOLOGY. 

THOMPSON,  SYLVANUS  P.:  A.    On  Binaural  Audition,  Phil.  Mag., 

Ser.  5,  IV.,  July-Dec.,   1877,  274-276;  VI.,  July-Dec.,  1878, 

383-391;  XII.,  July-Dec.,  1881,  351-355. 
B.  On  the  Function  of  the  Two  Ears  in  the  Perception  of  Space, 

Ibid.,  XIII.,  Jan.- June,  1882,  406-416. 
UKBANTSCHITSCH:  A.  Ueber  eine  Eigentiimlichkeit  der  Schallem- 

pfindungen  geringster  Intensitat,  Centralblatt  f.  d.  med.  Wis- 

sens.,  1875,  625-628. 

B.  Zur  Lehre  von  der  Schallempfindung,  Pfluger's  Archiv*  XXIV., 
1881,  574-595. 

C.  Ueber  das  An-    und    Abklingen    acustischer   Empfindungen, 
Ibid.,  XXV.,  1881,  323-342. 

WUNDT:  Work  cited  in  bibliography  of  Chap.  I.,  3teAufl.,  I.,  415 
ff.,  II. ,  42  ff.;  4te  Aufl.,  L,  443  ff. 

On  the  physics  and  physiology  of  sound,  reference  may  be  made,  in 
addition  to  the  works  already  mentioned,  to  Tyndall,  On 
Sound ;  Blaserna,  Theory  of  Sound  in  its  Eelations  to  Music ; 
Zahn,  Sound  and  Music;  and  Taylor,  Sound  and  Music.  The 
last  is  very  simple  and  untechnical,  and  is  perhaps  the  best  for 
those  approaching  the  subject  for  the  first  time. 

For  the  Stumpf-Wundt  discussion  on  pitch  distances  consult  the 
following:  Stumpf,  Zeitschrift  fur  Psychologic,  I.,  1890,  419; 
II.,  1891,  266,426,  438;  Engel,  Ibid,  II.,  1891,  361;  Wundt, 
Philos.  Studien,  VI.,  1890-91,  605;  VII.,  1891,  298,  633;  also 
Miinsterberg,  C,  above. 


104]  THE  MECHANISM  OF   THE  EYE.  89 


CHAPTER  V. 
The  Mechanism  of  the  Eye  and  Vision  in  General. 

THE  mechanism  of  the  eye  accomplishes  two  things  :  the 
projection  of  a  sharp  image  on  the  retina,  and  the  ready 
shifting  of  the  eye  so  as  to  bring  successive  portions  of  the 
image  into  the  best  position  for  seeing.  To  the  study  of 
these  mechanisms  and  other  physiological  phenomena  of 
importance  for  the  psychology  of  vision,  this  chapter  is 
devoted. 

THE  RETINAL  IMAGE  AND  ACCOMMODATION. 

104.  The  Retinal  Image.  This  is  easily  seen  in  the 
unpigmented  eye  of  a  pink-eyed  rabbit. 

a.  Chloroform  the  rabbit,  remove  the  eyes,  and  mount 
them  in  clay  for  readier  handling.  The  mounting  is  done 
as  follows  :  Make  a  thick  ring  of  clay  with  an  internal 
diameter  a  little  greater  than  that  of  the  cornea  of  the 
rabbit's  eye ;  place  the  eye,  cornea  downward,  in  the  ring ; 
lay  a  similar  ring  upon  it  to  keep  it  in  place,  and  press  the 
edges  of  the  rings  together.  The  eye  can  now  be  handled 
easily  and  turned  in  any  direction.  Turn  the  cornea 
toward  the  window,  and  observe,  from  behind,  the  inverted 
image  on  the  retina.  Bring  the  hand  into  range  and  move 
it  to  and  fro ;  observe  that  the  image  of  distant  objects  is 
more  distinct  than  that  of  the  hand.  The  dead  eye  is 
adjusted  for  distant  vision.  If  convex  and  concave  lenses 
are  at  hand  (spectacle  lenses  will  answer),  bring  them 
before  the  eye,  and  observe  that  the  effect  upon  the 


90  LABORATORY   COURSE  IN  PSYCHOLOGY.       [106 

retinal  image  is  similar  to  that  seen  subjectively  when 
they  are  held  before  the  observer's  own  eye,  provided 
that  that  is  normal. 

Reverse  the  eye,  holding  it  retina  side  toward  the  win- 
dow, and  observe  the  radiating  and  circular  fibres  of  the 
iris.  The  eye  must  be  fresh,  for  if  long  removed  it  loses 
its  transparency. 

105.  Accommodation.     The    sharpness    of   the    retinal 
image  depends  on  the  adjustment  of  the  crystalline  lens, 
which  must  be  such  as  to  focus  upon  the  retina  the  light 
from  the  object  under  regard.     The  lens  must  be  thicker 
and  rounder  for  near  objects,  thinner  and  natter  for  more 
distant  ones.     These  adaptations  of  the  eye  are  known  as 
Accommodation.     The  changes  in  the  clearness  of  the  retinal 
image  are  easy  to  observe  subjectively.     Hold  up  a  pin  or 
other  small  object  six  or  eight  inches  away  from  the  eyes. 
Close  one  eye  and  look  at  the  pin  with  the  other.     The  out- 
line of  the  pin  is  sharp,  but  the  outlines  of  things  on  the 
other  side  of  the  room  behind  it  are  blurred.     Look  at  these, 
and  the  outline  of  the  pin  becomes  blurred.     Notice  the 
feeling  of  greater  strain  when  looking  at  the  nearer  object. 
The  -experiment  is  somewhat  more  striking  when  the  nearer 
object  is  a  piece  of  veiling  or  wire  gauze,  and  the  farther,  a 
printed  page   held  at  such  a  distance  that  it  can  just  be 
read. 

On  this  and  the  next  two  experiments,  see  Helmholtz,  A,  112-118, 
Fr.  119-126  (90-96). 

106.  Schemer's  Experiment,     a.  Pierce  a  card  with  two 
fine  holes  separated  by  a  less  distance  than  the  diameter  of 
the  pupil,  say,  a  sixteenth  of  an  inch.     Set  up  two  pins  in 
corks,  distant  respectively  eight  and  twenty  inches  from 
the  eye  in  the  line  of  sight ;  close  one  eye,  and  holding  the 
card  close  before  the  other  with  the  holes  in  the  same  hori- 


106] 


THE  MECHANISM  OF   THE  EYE. 


91 


zontal  line,  look  at  the  nearer  pin;  the  farther  pin  will 
appear  double.  Look  again  at  the  nearer  pin,  and  while 
looking,  cover  one  of  the  holes  with  another  card ;  one  of 
the  images  of  the  farther  pin  will  disappear  —  the  left  when 
the  left  hole  is  covered,  and  the  right  when  the  right  is 
covered.  Look  at  the  farther  pin  or  beyond  it ;  the  nearer 
pin  appears  double.  Repeat  the  covering ;  closing  the  left 
hole  now  destroys  the  right  image,  and  covering  the  right 
destroys  the  left. 


Why  this  should  be  so  will  be  clear  from  the  diagrams 
above.  The  upper  diagram  illustrates  the  course  of  the  rays 
of  light  when-  the  eye  is  accommodated  for  the  nearer  pin ; 
the  lower  diagram  when  it  is  accommodated  for  the  farther 
pin.  A  and  B  represent  the  pins;  S  and  S  the  pierced 
screen;  d  and  d'  the  holes  in  the  screen ;-c  and  c  the  lens; 
of  I  a"  and  V  a  b"  the  retin'se ;  A',  A",  Bf  and  B",  tli3  positions 
of  the  double  images.  The  solid  lines  represent  the  course  of 
the  rays  from  the  pin  that  is  accommodated  for ;  the  lines 
of  short  dashes,  the  course  of  the  rays  from  the  other  pin  : 


92  LABORATORY  COURSE  IN  PSYCHOLOGY.       [107 

the  lines  of  long  dashes,  the  lines  of  direction ;  i.e.,  approxi- 
mately those  giving  the  direction  in  which  the  images 
appear  to  the  observer.  In  the  upper  diagram  the  rays 
from  B  are  focused  to  a  single  retinal  image  at  b,  while 
those  from  A,  being  less  divergent  at  first,  are  brought  to 
a  focus  nearer  the  lens,  cross  over  and  meet  the  retina  at 
a!  and  a!',  and,  since  each  hole  in  the  screen  suffices  to 
produce  an  image,  cause  the  pin  to  appear  double.  Its  two 
images  are  referred  outward  as  all  retinal  images  are,  along 
the  lines  of  direction  (which  cross  a  little  forward  of  the 
back  surface  of  the  lens,  in  the  crossing  point  of  the  lines  of 
direction),  the  right  retinal  image  corresponding  with  the 
left  of  the  double  inffeges  and  vice  versa.  If  now  the  right 
hole  (d)  in  the  screen  be  closed,  the  left  retinal  image  and 
the  right  double  image  disappear.  The  case  of  accommo- 
dation for  the  farther  pin  will  Be  clear  from  the  lower 
diagram,  if  attention  is  given  to  the  dotted  and  dashed 
lines.  It  will  also  be  easy  to  explain  why  moving  the  card 
when  looking  through  a  single  pin-hole  causes  apparent 
movements  of  the  pin  not  accommodated  for,  and  why  in 
one  case  the  movement  seems  to  be  with  the  card,  and  in  the 
other  case  against  it. 

b.  Stick  the  pins  into  the  corks  so  that  they  shall  extend 
horizontally,  and  examine  them  with  the  card  held  so  as  to 
bring  the  holes  one  above  the  other. 

c.  Arrange   the   holes   thus :  .  • .  and   observe   that   the 
triple  image  of  the  nearer  pin  (when  the  farther  is  fixated) 
has  the  reverse  figure  • .  • 

Schemer's  experiment  can  easily  be  illustrated  with  any 
convex  lens  and  a  pierced  screen  of  suitable  size. 

107.  Range  of  Accommodation,  a.  Find  by  trial  the 
nearest  point  at  which  a  pin  seen  as  in  Scheiner's  experi- 
ment can  be  seen  single.  This  is  the  near  point  of  accom- 


108]  THE  MECHANISM   OF   THE  EYE.  93 

modation.     For  the  short-sighted  a  far  point  may  also  be 
found,  beyond  which  double  images  reappear. 

b.  Find  how  far  apart  in  the  line  of  sight  two  pins  may 
be,  and  yet  both  be  seen  single  at  one  and  the  same  time. 
Try  with  the  nearer  at  20  cm.,  at  50  cm.,  at  2  m.  That 
portion  of  the  line  of  sight,  for  points  in  which  the  same 
degree  of  accommodation  is  sufficient,  is  called  the  Line  of 
Accommodation.  ,  The  length  of  the  line  increases  rapidly  as 
the  distance  of  the  object  from  the  eye  increases. 

Helmholtz,  A,  114,  119,  Fr.  122  (93),  128  (97). 

108.  Mechanism  of  Accommodation.  The  change  in  the 
lens  in  accommodation  is  chiefly  a  bulging  forward  of  its 
anterior  surface.  This  may  be  observed  as  follows  :  — 

a.  Let  the  subject  choose  a  far  and  a  near  point  of  fixation 
in  exactly  the  same  line  of  vision  ;  close  one  eye  and  fix  the 
other  upon  the  far  point.     Let  the  observer  place  himself 
so  that  he  sees  the  eye  of  the  subject  in  profile  with  about 
half  the  pupil  showing.     Let  the  subject  change  his  fixation 
at  request,  from  the  far  to  the  near  point,  and  vice  versa, 
being  careful  to  avoid  any   side  wise  motion   of  the   eye. 
The  observer  will  notice,  when  the  eye  is  accommodated  for 
the  near  point,  that  more  of  the  pupil  shows  and  that  the 
farther  side  of  the  iris  seems   narrower.     This  change  is 
due  to  the  bulging  forward  of  the  front  of  the  lens.     If 
the   change   were   due   to   accidental   turning  of   the   eye 
toward  the  observer,  the  farther  edge  of  the  iris   should 
appear  wider  instead  of   narrower.     Notice  also  that  the 
diameter  of  the  pupil  changes  with  the  accommodation. 

b.  Purkinje's  Images.     The  changes  in*the  curvature  of 
the  lens  may  also  be  observed  by  means  of   the  images 
reflected  from  its  front  and  back  surfaces  and  from  the  front 
of  the  cornea.     Operate  in  a  darkened  room.     Let  the  sub- 
ject choose  far  and  near  fixation  points  as  before.     Let  the 


94  LABORATORY  COURSE  IN  PSYCHOLOGY.       [108 

observer  bring  a  candle  near  the  eye  of  the  subject  at  a 
level  with  it  and  a  little  to  one  side,  and  place  his  own  eye 
in  a  position  symmetrical  to  the  candle  011  the  other  side  of 
the  subject's  line  of  sight.  Careful  examination  and  some 
shifting  about  of  the  place  of  the  candle  and  of  the  observer 
will  show  three  reflected  images  of  the  flame :  one  on  the 
side  of  the  pupil  next  the  light,  easily  recognizable,  bright 
and  erect,  reflected  from  the  surface  of  the  cornea ;  a  second, 
nearer  the  centre  of  the  pupil  and  apparently  the  farthest 
back  of  the  three,  erect  like  the  first,  but  very  indistinct 
(more  like  a  light  cloud  than  an  image),  reflected  from  the 
anterior  surface  of  the  lens ;  and  a  third,  a  mere  point  of 
light,  near  the  side  of  the  pupil  farthest  from  the  flame, 
inverted  and  reflected  from  the  posterior  surface  of  the 
lens.  When  the  observer  has  found  these  three  images,  the 
subject  should  fixate  alternately  the  near  and  far  points 
chosen.  As  he  fixates  the  near  point,  the  middle  image 
will  grow  smaller,  advance,  and  draw  toward  the  corneal 
image  ;  when  he  fixates  the  far  point,  the  image  will  enlarge, 
recede,  and  move  away  from  the  corneal  image.  The  follow- 
ing diagram,  after  Aubert,  illustrates  the  movement  of  the 

middle  image ;  the  full 
lines  indicate  the  posi- 
tions of  the  cornea  and 
lens  and  the  course  of 
the  rays  of  light  when 
the  eye  is  accommodated 
for  the  far  point ;  the 
dotted  lines  indicate 
the  anterior  surface  of 
the  lens  and  the  direc- 
tion of  the  ray  reflected 
from  its  surface  when  the  eye  is  accommodated  for  the  near 
point.  Three  images  similar  to  those  in  question  can  be 


109]  THE  MECHANISM  OF  THE  EYE.  95 

observed  on  a  watch  glass  and  a  double  convex  lens  held  in 
the  relation  of  the  cornea  and  crystalline.1 

Helmholtz,  A,  131-141,  especially  131-134,  Fr.  142-154  (104-112), 
especially  142-146  (104-107);  Aubert,  A,  444;  Tscherning. 

109.  Dioptrical  Defects  of  the  Eye.  Of  these  defects 
only  two  will  be  considered  here :  Astigmatism  and  Chro- 
matic Aberration.  The  first  is  an  error  in  the  form  or  set- 
ting of  the  refracting  surfaces,  which  prevents  their  bringing 
parallel  light  to  a  focus  in  a  single  point.  If  the  curvature 
of  the  lens,  for  example,  (or  of  the  cornea),  is  greater  on  the 
vertical  meridian  than  on  the  horizontal,  parallel  light  fall- 
ing upon  the  first  will  be  brought  to  a  focus  nearer  the  lens 
than  that  falling  upon  the  second.  This  makes  it  impossible 
for  the  astigmatic  eye  to  see  all  parts  of  a  plane  figure  with 
equal  distinctness  at  the  same  time.  Chromatic  Aberration 
depends  upon  the  different  degrees  of  refraction  which  dif- 
ferent colored  lights  experience  in  traversing  the  lens ; 
those  of  short  wave-length  (violet  and  blue)  are  most  re- 
fracted, those  of  long  wave-length  (red  and  orange)  least, 
and  the  others  in  order  between.  The  point  at  which  paral- 
lel violet  rays  are  brought  to  a  focus  is  therefore  nearer  the 
lens  than  the  point  for  red.  In  order,  therefore,  that  the 
same  degree  of  accommodation  may  serve  to  show  a  red 
lighted  object  and  a  violet  lighted  object  at  the  same  time 
and  both  with  full  distinctness,  the  red  light  must  be  less 
divergent  than  the  violet ;  in  other  words,  the  red  lighted 
object  must  be  somewhat  farther  away. 

a.  Astigmatism.  Make  a  fine  pin-hole  in  a  card ;  hold  it 
at  arm's  length  against  a  bright  background  and  accommo- 

1  By  using  a  magnifying-glass  a  second  faint  corneal  image  very  close  to  the 
first  can  be  seen,  when  the  light  strikes  the  cornea  well  toward  one  side.  When 
this  is  counted,  as  it  is  by  Tscherning,  there  are  four  Purkinje  images,  those  from 
the  front  and  back  of  the  lens  becoming  the  third  and  fourth  in  the  enumeration, 
instead  of  the  second  and  third. 


96 


LABORATORY  COURSE   IN  PSYCHOLOGY.      [110 


date  the  eye  for  a  nearer  point,  or  put  on  convex  glasses. 
The  spot  will  not  appear  as  a  little  circle  of  light,  as  it 
would  if  the  lens  and  cornea  were  perfect  in  form,  but  as  a 
more  or  less  irregular  star  or  flower-shaped  figure  in  which 
portions  of  several  images  of  the  hole  may  be  made  out. 
Accommodate  for  a  point  considerably  beyond  the  card 
and  notice  the  change  in  the  figure. 

These  irregularities  (phenomena  of  Irregular  Astigma- 
tism) disappear,  however,  with  exact  accommodation,  but 
another  kind  (Regular  Astigmatism)  is  then  to  be  observed. 
Close  one  eye  and  look  with  the  other  at  the  centre  of  the 
radiating  figure  below.  Notice  which  lines  appear  with 
greatest  blackness  and  distinctness.  Try  the  effect  of 
increasing  and  decreasing  the  distance.  Try  also  the  other 
eye. 


Something  of  the  same  kind  is  to  be  seen  in  the  set  of 
concentric  circles ;  also  evidences  of  irregular  astigmatism 
when  accommodation  is  changed  or  when  the  distance  of 
the  diagram  is  increased  or  decreased.  Notice  especially 
the  rayed  appearance  and  the  distortion  of  the  inner  circles 
when  the  eye  is  accommodated  for  a  greater  distance  than 


110]  THE  MECHANISM  OF  THE  EYE.  97 

that  of  the  diagram.  On  the  latter  peculiarity,  see  von 
Bezold. 

b.  Chromatic  Aberration.  Bend  a  fine  platinum  wire 
into  a  ring  half  an  inch  in  diameter,  and  heat  it  white  hot 
in  the  flame  of  a  Buiisen  burner.  Look  at  the  ring  through 
a  pin-hole  in  a  black  card  held  at  such  a  distance  that  the 
ring  lies  close  to  the  edge  of  the  field  of  the  pin-hole  all 
around.  Accommodate  the  eye  for  the  centre  of  the  ring, 
and  observe  that  the  outer  edge  of  the  ring  appears  bright 
red,  the  inner  edge  blue  or  violet.  Substitute  for  the  card 
a  bit  of  blue  glass,  and  accommodate  first  for  the  glass,  then 
for  a  point  some  distance  beyond  the  ring.  In  the  first 
case  the  outer  and  inner  edges  of  the  ring  (except  as  astig- 
matism interferes)  will  both  be  blue ;  in  the  second  case 
they  will  be  red.  The  ordinary  blue  glass  allows  both  red 
and  blue  light  to  pass  through  it. 

Look  at  the  edge  of  the  window  frame  next  the  pane, 
and  bring  a  card  before  the  eye  so  that  about  half  the 
pupil  is  covered  ;  if  the  card  has  been  brought  up  from  the 
frame  side,  the  frame  will  be  bordered  with  yellow ;  if 
from  the  pane  side,  with  blue.  In  ordinary  vision  these 
fringes  do  not  appear,  because  the  colors  partially  overlap 
and  produce  a  practically  colorless  mixture. 

Yon  Bezold's  Experiment.  Look  at  the  parallel  lines  of 
the  left  figure  in  Ex.  118  with  imperfect  accommodation, 
e.g.,  through  convex  spectacles,  and  observe  the  aberra- 
tion colors.  If  a  set  of  heavy  concentric  circles  (separated 
by  equal  spaces,  and  beginning  with  a  central  black  dot 
of  a  diameter  equal  to  the  width  of  the  lines)  is  used 
instead  of  the  straight  line  figure,  it  will  be  possible  by 
changing  its  distance  from  the  eye  to  find  a  position  in 
which  the  aberration  colors  so  overlap  that  dark  and  light 
seem  to  have  changed  places,  and  the  central  spot  is  light 
instead  of  dark.  The  spiral  figure  with  Ex.  128  will  show 


98  LA130RATOHY  COURSE  IN  PSYCHOLOGY.    [llO 

something  of  the  effect,  but  the  central  black  spot  is  too 
large  to  show  it  completely. 

Both  astigmatic  differences  and  the  aberration  colors  may 
at  times  influence  judgments  of  distance. 

On  a,  Helmholtz,  A,  169  ff.,  Fr.  187  (138)  ff.  On  6,  Helmholtz 
A,  156-164,  Fr.  172-179  (125-131);  von  Bezold;  Tumlirz. 

ENTOPTIC  APPEARANCES. 

110.  Floating  Particles  in  the  Media  of  the  Eye  and  on 
its  Surface ;  Muscce  Volitantes.  Fix  a  lens  of  short  focus 
at  some  distance  from  a  bright  gas  or  candle  flame.  Set  up 
in  the  focus  of  the  lens  a  card  pierced  with  a  very  fine  hole ; 
bring  the  eye  close  to  the  hole  and  look  toward  the  light. 
The  eye  should  be  far  enough  from  the  hole  to  prevent  the 
edge  of  the  lens  from  being  seen.  The  rays  of  light  that 
now  reach  the  eye  are  strongly  divergent,  and  the  crystalline 
lens  does  not  bring  them  to  a  focus  on  the  retina,  but  only 
refracts  them  to  such  a  degree  that  they  traverse  the  eye 
nearly  parallel,  and  thus  in  suitable  condition  for  casting 
sharp  shadows  upon  the  retina  of  objects  on  or  in  the  eye. 

a.  The  lens  will  appear  full  of  light,  and  in  it  will  be 
seen  a  variety  of  shadings,  blotches,  and  specks,  single  or  in 
strings,  the  outward  projection  of  the  shadows  just  men- 
tioned. The  figures  in  this  luminous  field  will  vary  from 
person  to  person,  even  from  eye  to  eye,  but  in  almost  every 
eye  some  will  be  found  that  move  and  some  that  remain 
fixed  or  only  move  with  the  eye.  Of  the  moving  figures 
some  are  due  to  particles  and  viscous  fluids  on  the  surface 
of  the  eye  ;  they  seem  to  move  downward,  and  are  changed 
by  winking.  Notice,  for  example,  the  horizontal  bands  that 
follow  a  slow  dropping  and  raising  of  the  upper  lid.  Such 
appearances  as  these,  since  their  cause  is  not  really  in  the 
eye  but  outside  of  it,  have  been  called  pseudentoptic  by 
Laqueur.  Others,  the  muscce  volitantes,  are  frequently 


Ill]  THE  MECHANISM  OF  THE   EYE.  99 

noticed  without  any  apparatus  ;  they  appear  as  bright 
irregular  threads,  strings  of  beads,  groups  of  points,  or 
single  minute  circles  with  light  centres.  They  seem  to 
move  downward  in  the  field,  but  actually  move  upward  in 
the  vitreous  humor  where  they  are  found.  Of  the  per- 
manent figures,  some  "are  due  to  irregularities  of  structure 
or  small  bodies  in  the  crystalline  and  its  capsule  (spots 
with  dark  or  bright  centres,  bright  irregular  lines,  or 
dark  radiating  lines  corresponding  probably  to  the  radial 
structure  of  the  lens) ;  others  of  a  relatively  permanent 
character,  it  is  said,  can  be  produced  on  the  cornea  by 
continued  rubbing  or  pressure  on  the  eyeball.  . 

b.  The  round  spot  of  light  in  which  these  things  are  seen 
represents  the  pupil,  and  the  dark  ground  around  it  is  the 
shadow  of  the  iris.  Notice  the  change  in  the  size  of  the 
spot  of  light,  as  the  eye  is  accommodated  for  different  dis- 
tances (cf.  Ex.  108),  or  as  the  other  eye  is  exposed  to,  or 
covered  from,  the  light.  The  change  begins  in  about  half 
a  second.  It  shows  the  close  connection  of  the  iris 
mechanisms  of  the  two  eyes,  and  is  typical  of  the  way  in 
which  the  two  eyes  co-operate  as  parts  of  a  single  visual 
organ. 

Some  of  these  entoptic  observations  may  be  made  with  a 
pierced  card  alone,  or  simply  by  looking  directly  at  a  broad 
expanse  of  clear  sky  without  any  apparatus  at  all. 

Helmholtz,  A,  184-192,  and  Tafel  I.,  which  shows  the  appearance 
of  several  of  these  entoptic  objects,  Fr.  204-214  (149-156)  and  PI.  V., 
also  548-558  (419-427) ;  Laqueur. 

111.  Eetinal  Blood-vessels,  Purkinje's  Vessel  Figures. 
a.  Concentrate  a  strong  light  (preferably  in  a  dark  room), 
or  even  direct  sunlight,  with  a  double  convex  lens  of  short 
focus  on  the  sclerotic  in  the  outer  corner  of  the  eye  of  the 
subject,  requesting  him  to  turn  the  eye  toward  the  nose 


100         LABORATORY  COURSE  IN  PSYCHOLOGY,     [ill 

and  giving  him  a  dark  background  to  look  toward.  Mane 
the  spot  of  light  on  the  sclerotic  as  small  and  sharp  as 
possible,  and  give  to  the  lens  a  gentle  to  and  fro  or  circular 
motion.  After  a  little  the  subject  will  see  upon  the  field, 
which  the  light  makes  reddish-yellow,  the  dark  branching 
figure  of  the  shadows  of  the  retinal  vessels.  Notice  that 
the  spot  directly  looked  at  is  partially  surrounded,  but  not 
crossed,  by  the  vessels.  In  this  lies  the  yellow  spot  (macula 
lutea),  the  retinal  area  of  clearest  vision.  The  centre  from 
which  the  vessels  radiate  lies  in  the  point  of  entrance  of 
the  optic  nerve.  In  this  form  of  the  experiment  the  light 
radiates  in  all  directions  within  the  eye  from  the  illumi- 
nated point  of  the  sclerotic. 

b.  Somewhat  the  same  sort  of  image  is  to  be  secured  by 
moving  a  candle  about  near  the  eye,  below  it  and  a  little  to 
one  side.     In  this  experiment  some  indication  of  the  region 
of  the  yellow  spot  is  to  be  seen.     This  time  the  light  enters 
by  the  pupil,  forms  an  image  on  a  part  of  the  retina  some- 
what  remote  from  the   centre,  and  this   retinal  image  is 
itself  the  source  of  the  light  by  which  the  vessel  shadows 
are  cast. 

c.  Look  through  a  pin-hole  in  a  card,  held  close  before 
the  eye,  at  the  sky  or  some  other  illuminated  surface,  or  at 
a  broad  gas-flame.     Give  the  card  a  rather  rapid  circular 
motion,  and  the  finer  retinal  vessels  in  the  region  of  the 
yellow  spot  will  readily  be  seen,  among  them  also  a  small 
colored  or  slightly  tinted  spot  (best  seen,  perhaps,  by  gas- 
light) representing  the  macula,  and  in  its  centre  a  shadowy 
dot  (representing  the  fovea,  the  point  of  clearest  vision), 
which  appears  to  rotate  when  the  motion  of  the  card  is 
circular.     If  the  card  is  moved  horizontally,  the  vertical 
vessels  alone  appear;  if  vertically,  the  horizontal  vessels. 
Notice  also  the  granular  appearance  of  the  macula;   the 
granulations  have  been  supposed  to  represent   the  visual 


OF 


112]  "    THE  MECHANISM  OF  THE  EYE.  101 

cones  of  that  region.  The  finer  retinal  vessels  can  also  be 
seen  when  looking  at  the  vacant  field  of  a  compound  micro- 
scope, if  the  eye  is  moved  about  rapidly. 

In  all  cases  it  is  important  that  the  shadows  be  kept 
moving  ;  if  they  stand  still,  they  are  lost.  The  explanation 
is  partly  physiological  (the  portions  of  the  retina  on  which 
the  shadows  rest  soon  gain  in  sensitiveness  enough  to  com- 
pensate for  the  less  light  received)  and  partly  psychological 
(moving  objects  in  general  arouse  spontaneous  attention,  and 
those  whose  images  rest  continuously  on  the  retina  without 
motion  are  particularly  subject  to  neglect). 

Once  having  become  familiar  with  these  vessel  figures,  it 
is  often  possible  for  the  observer  to  see  traces  of  them 
without  any  apparatus.  Parts  of  them,  with  something  of 
the  yellow  spot,  may  sometimes  be  seen  for  an  instant  as 
dark  figures  on  the  diffusely  lighted  walls  and  ceiling,  or  as 
light  figures  on  the  dark  field  of  the  closed  eyes,  when  the 
eyes  are  opened  and  closed  after  a  glance  at  the  window  on 
first  waking  in  the  morning,  or  as  blue  figures  when  looking 
at  the  snow  and  winking  on  a  bright  winter  morning. 

Helmholtz,  A,  192-198,  555,Fr.  214-221  (156-161),  528  (402). 

112.  Retinal  Circulation.  Look  steadily  through  two  or 
three  thicknesses  of  blue  glass  at  the  clear  sky  or  a  bright 
cloud,  and  observe  the  bright  points  darting  hither  and 
thither  like  bees  in  a  swarm  or  snowflakes  on  a  windy  day. 
Careful  observation  will  also  establish  that  the  bright  points 
are  followed  by  shadowy  darker  ones.  Pick  out  a  speck  on 
the  window  to  steady  the  eyes,  and  observe  that  while  the 
movements  of  the  points  seem  irregular  the  same  lines  are 
retraced  by  them  from  time  to  time.  When  several  of 
their  courses  have  been  accurately  determined  for  one  of 
the  eyes,  repeat  the  experiment  for  demonstrating  the  finer 
retinal  vessels  (Ex.  Ill,  c),  and  notice  that  fine  vessels 


102       LABORATORY  COURSE  IN  PSYCHOLOGY.      [113 

are  found  which  correspond  to  the  courses  that  the  points 
seem  to  follow.  These  flying  points  can  be  seen  without 
the  glass  by  a  steady  gaze  at  an  evenly  lighted  bright  sur- 
face, and  sometimes  a  rhythmical  acceleration  of  their 
movements  will  be  found,  corresponding  to  the  pulse. 
Helmholtz  explains  the  phenomenon  by  a  temporary  clog- 
ging of  fine  capillary  vessels  by  large  blood  corpuscles. 
The  bright  lines  (the  apparent  tracks  of  bright  points)  are 
really  the  relatively  empty  capillary  tubes  ahead  of  the  cor- 
puscles, which,  after  an  instant,  are  driven  onward  by  others 
crowding  behind,  which  in  turn  give  the  shadow  that  ap- 
parently follows  the  bright  points. 

Helmholtz,  A,  198  f.,  Fr.  221  (837),  555  (425);  Rood.      ; 

113.  The  Blind  Spot.  Mariotte's  Experiment.  The  point 
of  entrance  of  the  optic  nerve  is  unprovided  with  visual 
end-organs  and  is  irresponsive  to  light.  This  insensitive- 
ness  is  easily  demonstrated  with  the  diagrams  below. 


a.  Close  the  left  eye,  and  keeping  the  right  fixed  on  the 
upper  asterisk  in  the  diagram  move  the  latter  toward  the 
eye  and  away  from  it  till  a  point  is  found  where  the  black 
oval  disappears.  For  the  blind  spot  of  the  left  eye,  turn 
the  diagram  upside  down  and  close  the  right  eye. 

The  blind  spot  may  be  demonstrated  simultaneously  in 
both  eyes  with  the  figure  on  the  next  page.  The  experi- 
menter should  look  at  the  asterisk  while  he  holds  a  card 


114]  THE  MECHANISM  OF  THE  EYE.  103 

in  the  median  plane   of  his   head,  to   prevent  each  eye 
from  seeing  the  other's  part  of  the  diagram. 

O  O 

b.  To  draw  the  projection  of  the  blind  spot,  arrange  a 
head-rest  opposite  a  vertical  sheet  of  white  paper,  and  15  or 
18  inches  distant  from  it.  Put  a  dot  on  the  paper  for  a 
fixation  point.  Fasten  upon  the  end  of  a  light  rod  a  bit  of 
black  paper  about  2  mm.  square  or  blacken  the  end  of  the 
rod  with  ink.  Bring  the  face  into  position,  ctyse  one  eye, 
and  fix  the  other  upon  the  dot.  Move  the  rod  slowly  so  ac 
to  bring  the  little  square  over  the  part  of  the  paper  corre- 
sponding to  the  blind  spot,  dotting  on  the  paper  the  points 
where  the  square  disappears  or  reappears.  Repeat  at  vari- 
ous points  till  the  outline  of  the  projection  of  the  blind  spot 
is  complete.  If  the  mapping  is  carefully  carried  out,  the 
map  will  probably  also  show  the  points  of  departure  of  the 
large  blood-vessels  that  enter  with  the  nerve. 

Helinholtz,  A,  250-254,  Fr.  284-289  (210-214). 


114.  The  Filling-out  of  the  Blind  Spot  is  of  considerable 
psychological  interest.  The  mind  supplies  what  is  lacking 
in  the  sense,  and  in  doing  so  is  influenced  both  by  the  sensa- 
tions of  the  parts  of  the  retina  surrounding  the  spot  and 
by  previous  experience.  In  ordinary  two-eyed  vision  the 
blind  spot  of  one  eye  corresponds  to  a  seeing  spot  in  the 
other,  and  this  with  the  movements  of  the  eyes  amply  sup- 
plies the  defect.  The  spot,  furthermore,  lies  so  far  out 
of  the  range  of  clear  vision  that  its  existence  is  habitually 
overlooked,  even  in  monocular  vision. 

a.   When  the  image  of  the  oval  in  a  of  the  last  experi- 


104         LABORATORY  COURSE  IN  PSYCHOLOGY.      [114 

ment  is  brought  wholly  upon  the  spot,  the  paper  seems  an 
unbroken  white,  because  the  adjacent  parts  of  the  retina 
are  stimulated  with  white.  When,  however,  the  diagram 
is  held  a  little  nearer  so  that  the  edge  of  the  black  oval  can 
be  seen,  the  filling  is  part  black  and  part  white. 

b.  The  effect  of  experience  appears  when  the  oval  is 
replaced  by  such  a  figure  as  that  below,  or  any  other  in 
which  the  bars  stand  out  well  from  one  another  and  the 
background. 


When  the  image  of  the  middle  of  this  diagram  falls  upon 
the  blind  spot,  one  bar  will  seem  to  cross  completely  over 
the  other.  Bars  that  cross  are  so  much  more  frequent  in 
experience  than  those  that  are  mitered  together  that  the 
sensations  of  the  adjacent  parts  are  thus  interpreted. 
Skill  in  observation  in  indirect  vision  seems  to  hinder  this 
filling-out  process  somewhat,  probably  by  aiding  in  more 
exact  distinguishing  of  the  character  of  the  sensations 
received.  Both  Helmholtz  and  Aubert  find  themselves 
unable  to  determine  how  the  parts  of  the  figure  resting  on 
the  blind  spot  are  related. 

Helmholtz,  .1,  Fr.  734-745  (574-583);  Aubert,  .1,  505. 


116]  THE  MECHANISM  OF  THE  EYE.  105 

115.  The  Yellow  Spot,  the  Macula  Lutea.    The  projection 
of  the  yellow  spot  in  the  visual  field  can  be  made  visible  in 
several  ways.     Two  have  already  been  mentioned  in  Ex. 
Ill ;    others   are   as   follows :    Close   the   eyes   for   a   few 
seconds  and  then  look  through  a  flat-sided  bottle  of  chrome 
alum  solution  at  a  brightly  lighted  surface  or  at  the  clear 
sky.     In  the  blue-green  solution  a  rose-colored  spot  will  be 
seen  which  corresponds  to  the  yellow  spot.     The  light  that 
comes  through  the  chrome  alum  solution  is  chiefly  a  mixture 
of  red  and  green  and  blue.     The  pigment  of  the  yellow  spot 
absorbs  a  portion  of  the  blue  and  green  and  transmits  the 
rest,  which   makes  a  rose-colored   mixture,  to    the  visual 
organs  behind  it.     The  same  can  be  very  beautifully  de- 
monstrated with  violet  or  purple  gelatine  sheets. 

Helmlioltz,  A,  Fr.  548-551  (419-421);  Maxwell;  Sachs;  Hering,  C. 

116.  Intermittent  Illumination.     The  region  of  the  yel- 
low spot  can  be  seen,  together  with  many  other   curious 
figures  and  patterns,  when  the  illumination  of  a  single  eye 
is  made  intermittent  by  moving  the  spread  fingers  rapidly 
to  and  fro  before  it.     Something  may  be  seen  when  the 
open  eyes  are  fixed  on  a  uniformly  lighted  surface,  but  more 
when  they  are  turned  with  closed  lids  toward  a  bright  sky 
or  the  sun  itself.     The  figures  probably  differ  in  different 
eyes  and  some  are  beautiful  and  elaborate.     Sometimes  with 
steady  fixation  the  figures  give  place  more  or  less  completely 
to  a  general  streaming  of  fine  particles,  suggesting  the  flying 
specks  of  Ex.  112,  but  finer  and  of  less  regular  course. 
Vierordt  credited  the  appearance  to  the  circulation  of  the 
blood  in  the  retinal  vessels ;  Helmholtz  is  inclined  to  think 
the  fine  particles  lymph  corpuscles  rather  than  blood  corpus- 
cles.    Similar  phenomena  are  to  be  observed  with  black  and 
white  disks  when  rotated  at  less  speed  than  that  required 
for  uniform  mixing  of  the  black  and  white. 

Helmholtz,  A,  532  f.,  Fr.  502  (381)  f.;  Exner,  F. 


106        LABOEATOEY  COUESE  IN  PSYCHOLOGY.       [117 

117.  Acuteness  of  Vision,  Minimum  Visibile.  a.  Place 
the  parallel  line  diagram  used  in  Ex.  118  in  a  good  light 
and  walk  backward  from  it  till  the  lines  can  just  no  longer 
be  distinguished  as  separate.  If  the  experimenter's  eyes  are 
not  normal,  he  should  use  glasses  that  fit  his  eyes  for  dis- 
tinct vision  at  the  distance  required.  Measure  the  distance 
between  the  eye  and  the  diagram,  and  calculate  the  angle 
whose  apex  lies  in  the  crossing  point  of  the  lines  of  direc- 
tion (about  7.2  mm.  back  of  the  cornea  and  15.6  mm.  in 
front  of  the  retina)  and  whose  base  is  the  distance  from 
the  middle  of  one  line  of  the  diagram  to  the  middle  of  the 
next;  in  this  diagram  1.6  mm.  This  angle  measures  the 
least  visible  extent  when  discrimination  is  involved ;  the 
least  luminous  extent  that  can  still  impress  the  retina  is  far 
smaller,  as  witness  the  visibility  of  the  stars.  On  the  sup- 
position that  if  the  sensations  of  two  cones  are  to  be  separ- 
able they  must  be  separated  by  an  unstimulated  cone,  or  at 
least  by  a  less  stimulated  one,  it  has  generally  been  consid- 
ered that  the  cones  could  not  subtend  a  greater  angle 
than  that  found  in  this  experiment,  60" — 90",  represent- 
ing 0.004 — 0.006  mm.  on  the  retina,  and  this  agrees 
well  with  microscopical  measurements.  But  as  Helm- 
holtz  notices  (Phys.  Opt.,  2d  ed.,  p.  260),  this  experiment 
does  no  more  than  prove  that  there  are  on  the  retina  rows 
of  sensitive  elements,  the  middle  lines  of  which  are  sepa- 
rated by  the  angular  distance  found  in  the  experiment. 
The  elements  themselves,  if  properly  arranged,  may  be 
somewhat  larger.  Calculation  of  the  number  of  such  ele- 
ments in  a  sq.  mm.  of  the  retina,  based  on  this  view  of  the 
experiment,  agrees  well  in  the  case  of  Helmholtz's  own 
determination  with  the  result  of  microscopical  counting. 

b.  The  discriminative  power  of  the  retina  falls  off  rapidly 
in  all  directions  from  the  fovea  —  more  rapidly  above  and 
below  than  in  a  horizontal  direction.  Arrange  a  head-rest 


118]  THE  MECHANISM  OF  THE  EYE.  107 

and  perpendicular  plane  a's  in  Ex.  113,  b  (or  if  a  perimeter 
is  at  hand  use  that).  Place  upon  the  end  of  the  rod  used  in 
that  experiment  a  card  on  which  have  been  made  two  black 
dots  2  mm.  in  diameter  and  4  mm.  from  centre  to  centre. 
Move  the  card  horizontally  toward  the  fixation  point,  begin- 
ning beyond  the  point  at  which  the  two  dots  can  be  dis- 
tinguished and  moving  inward  till  they  can  just  be 
distinguished.  Measure  the  distance  from  the  fixation 
point,  and  repeat  several  times  both  to  the  right  and  left 
of  the  fixation  point,  holding  the  card  so  that  both  dots  are 
in  each  case  equally  distant  from  that  point.  Try  the  same 
for  the  vertical  meridian. 

Helmholtz,  A,  255-264,  Fr.  291-301  (215-223);  Uhthoff.     On  a, 
Aubert,  A,  579-585;  on  6,  585-591.     On  6,  see  also  Exner,  Z>,  242  ff. 


118.  Bergmann's  Experiment.  Place  the  left  hand  dia- 
gram in  a  good  light,  and  look  at  it  from  a  distance  of  a  yard 
and  a  half  or  two  yards.  Observe  the  apparent  bending  and 
beading  of  the  lines.  This  is  believed  by  Helmholtz  to  be  due 
to  the  mosaic  arrangement  of  the  visual  cones.  The  cones 
that  are  touched  by  the  image  of  one  of  the  white  lines  are 
stimulated  in  proportion  as  they  are  more  or  less  touched. 
Those  that  are  much  stimulated  furnish  the  sensation  of 
the  white  line  and  its  irregularities;  those  that  are  little 


108       LABORATORY  COURSE  IN  PSYCHOLOGY.       [119 

stimulated  join  with  those  that  are  not  touched  at  all  to 
give  the  image  of  the  black  line  and  its  irregularities. 
This  is  schematically  represented  in  the  right  hand  cut. 
Von  Fleischl,  on  the  other  hand,  has  made  experiments  to 
show  that  the  bending  and  beading  of  the  lines  is  not  con- 
nected with  the  retinal  mosaic,  but  rather  with  movements 
of  the  eyes  that  sweep  the  point  of  fixation  backward  and 
forward  across  the  lines.  Further  than  this  his  explanation 
does  not  go. 

Helmholtz,  A,  257-258,  Fr.  293  294  (217-218) ;  von  Fleischl. 

119.  Mechanical  Stimulation  of  the  Retina,  a.  Phos- 
phenes.  Turn  the  open  or  closed  eye  as  far  as  possible 
toward  the  nose  and  press  on  the  eyelid  at  the  outer  corner 
with  the  finger  or  the  tip  of  a  penholder.  On  the  opposite 
side  of  the  visual  field  will  be  seen  a  more  or  less  complete 
circle  of  light  surrounded  by  a  narrow  dark  band,  outside  of 
which  again  is  a  narrow  band  of  light.  Notice  the  color  of 
the  light  seen.  Get  phosphenes  by  pressure  at  other  points 
of  the  eyeball. 

b.  Press  the  eye  moderately  with  some  large  object,  say, 
the  angle  of  the  wrist  when  the  hand  is  bent  backward,  and 
continue  the  pressure  for  a  minute  or  two.  Peculiar  palpi- 
tating figures  will  be  observed  and  strange  color  effects. 
The  former  Helmholtz  compares  to  the  tingling  of  a  mem- 
ber that  is  "  asleep." 

e.  Standing  before  a  window,  close  the  eyes  and  turn 
them  sharply  from  side  to  side.  As  they  reach  the  extreme 
position  in  either  direction,  observe  immediately  in  front  of 
the  face  a  sudden  blue  spot  surrounded  by  a  yellow  band. 
A  second  fainter  spot  farther  from  the  centre  in  the  direc- 
tion of  motion  may  also  be  seen.  The  appearance  of  the 
first  spot  is  due  to  a  mechanical  stimulation  of  a  portion  of 
the  retina  at  the  edge  of  the  blind  spot  in  the  eye  that  turns 


121]  THE  MECHANISM  OF  THE  EYE.  109 

inward.     The  second  spot  belongs  to  the  corresponding  area 
in  the  other  eye. 

Helmholtz,  A,  235-239,  Fr.  266-270  (196-200),  744  (583)  f. 

120.  Idio-retinal  Light,  Light  Chaos,  Light  Bust.    a.  Close 
and  cover  the  eyes  so  as  to  exclude  all  light,  taking  care 
not  to  press  them,  or  experiment  in  a  perfectly  dark  room. 
Let  the  after-effects  of  objective  light  fade  away,  and  then 
watch  the  shifting  clouds  of  retinal  light.     The  cause  of 
the  retinal  light  is  not  altogether  clear,  but  it  is  supposed 
to  be  a  chemical  action  of  the  blood  on  the  nervous  portion 
of  the  visual  apparatus.     Aubert  estimates  its  brightness 
at  about  half  the  brightness  of  a  sheet  of  paper  illuminated 
by  the  planet  Venus  when  at  its  brightest. 

b.  When  awake  in  the  night  time  in  a  room  that  is  almost 
perfectly  dark  (e.g.,  in  which  the  form  of  the  window  and 
the  large  pieces  of  furniture  cannot  be  made  out),  notice 
that  the  white  clothing  of  the  arms  can  be  seen  faintly 
when  they  are  moved  about,  but  not  when  they  are  still.  In 
the  last  case  the  very  faint  light  they  reflect  is  not  sufficient 
to  make  them  distinguishable  from  clouds  of  idio-retinal 
light. 

Helmholtz,  A,  242-243,  Fr.  274-275  (202-203).  On  6,  Helm- 
holtz, B. 

121.  Electrical    Stimulation   of   the   Visual   Apparatus. 
Moisten  thoroughly  with  salt  water  both  the  electrodes  and 
the  portions  of  the  skin  to  which  they  are  to  be  applied. 
Place  one  of  the  electrodes  on  the  forehead  (or  on  the  edge 
of  the  table  and  lay  the  forehead  upon  it),  the  other  on  the 
back  of  the  neck ;.  or,  if  the  current  is  strong  enough,  hold 
it  in  the  hand  or  lay  it  on  the  table  with  the  hand  upon  it. 
At  each  opening  or  closing  of  the  circuit,  a  bright  flash  will 
be  seen,  whether  the  eyes  are  closed  or  open.   With  the  eyes 
closed  and  covered,  the  effects  of  the  continuous  current 


110       LABORATORY  COURSE  IN  PSYCHOLOGY.      [122 

may  be  observed.  In  this  case  it  is  well  to  apply  the  elec- 
trode slowly  and  carefully  so  as  to  avoid  as  much  as  possi- 
ble the  flash  caused  by  the  sudden  closing  of  the  circuit. 
When  the  positive  electrode  is  on  the  forehead,  the  nega- 
tive on  the  back  of  the  neck,  a  transient  pale  violet  light 
will  be  seen  distributed  generally  over  the  field  and  forming 
a  small  bright  spot  at  its  centre.  Sometimes  traces  of  the 
blind  spot  also  appear.  The  violet  light  soon  fades,  and  on 
opening  the  circuit  there  is  a  notable  darkening  of  the 
field,  with  a  momentary  view  of  the  blind  spots  as  bright 
disks.  When  the  negative  electrode  is  on  the  forehead,  the 
positive  on  the  back  of  the  neck,  the  phenomena  are  in 
general  reversed,  the  darkening  occurring  on  closing  the  cir- 
cuit, the  violet  light  on  opening  it.  Helmholtz  sums  up 
these  and  other  experiments  in  the  following  law :  "  Con- 
stant electrical  circulation  through  the  retina  from  the 
cones  toward  the  ganglion  cells  gives  the  sensation  of 
darkness ;  circulation  in  the  contrary  direction  gives  the 
sensation  of  brightness."  QPhys.  Opt.,  2d  ed.,  p.  247.)  That 
the  blind  spot  should  appear  as  a  disk  of  different  color 
from  the  rest  of  the  field  seems  to  be  due  to  the  fact  that 
the  sensitive  parts  of  the  retina  immediately  surrounding 
it  are  somewhat  shielded  from  the  electric  current,  and  as 
usual  their  condition  is  attributed  to  the  blind  spot  also. 
The  experiment  is  not  altogether  a  pleasant  one,  on  account 
of  the  feeling  which  the  current  produces  in  the  head,  the 
"  electrical  taste  "  in  the  mouth,  and  the  reddening  of  the 
skin  under  the  electrodes. 

Helmholtz,  A,  243-248,  Fr.  275-281  (203-207),  744  (583). 

RETINAL  FATIGUE  AND  ADAPTATION. 

122.  Retinal  Fatigue.  Stare  with  perfectly  fixed  and 
motionless  eyes  at  a  selected  spot  on  a  variegated  carpet  or 
wall  paper,  and  notice  the  levelling  effect  of  fatigue.  The 


123]  THE  MECHANISM  OF  THE  EYE.  Ill 

differences  in  color  and  pattern  gradually  disappear,  and 
the  whole  field  becomes  a  nearly  uniform  cloud.  The  parts 
of  the  recina  that  are  strongly  stimulated  are  brought 
down  to  the  general  level ;  those  that  are  little  stimulated 
are  built  up  to  it.  Every  wink  or  slight  movement  of  the 
eyes  causes  a  general  brightening  up  of  the  field  and 
restoration  of  vision.  The  experiment  is  particularly  easy 
to  make  when  looking  at  a  uniform  surface  with  faint 
shadows  lying  on  it. 

Helmholtz,  A,  508,  555  ff.,  Fr.  478  (362),  527  (402)  ff.;  Fick,  B, 
222;  Treitel;  Hering,  C.  See  also  the  discussion  on  this  topic  toy 
A.  E.  Fick  and  Hering. 

123.  Adaptation  of  the  Eye.  a.  The  adjustment  of  the 
eye  to  the  intensity  of  its  illumination  is  effected  partly  by 
change  in  the  size  of  the  pupil,  and  partly  by  changes  in 
the  retina  itself.  The  first  is  of  common  observation,  and 
the  connection  of  the  two  eyes  in  this  respect  has  been 
noticed  in  Ex.  110,  b.  The  effects  of  going  from  a  dark 
room  into  a  light  room  and  vice  versa,  and  the  gradual  im- 
provement of  vision  on  remaining  in  one  or  the  other,  are 
also  familiar. 

b.  It  has  not,  however,  been  so  generally  observed  that 
adaptation  to  very  weak  lights  is  much  more  favorable  to 
the  perception  of  colorless  light  than  to  colored.  This  may 
easily  be  observed  in  a  dark  room  with  single  flashes  of  a 
rather  faint  Geissler  tube.  Before  the  room  is  darkened, 
and  for  a  short  time  after,  the  colors  of  the  light  are  readily 
perceived.  After  some  time,  however,  they  nearly  or  quite 
fail,  seeming  to  be  lost  in  the  increased  brilliancy  of  the 
white  light.  It  is  important  that  there  should  be  an  inter- 
val between  the  flashes  sufficient  to  allow  all  the  effects  of 
one  to  disappear  before  another,  is  given.  If  the  room*  is 
not  completely  dark,  the  heatt  of  the  observer  and  the  tube 


112        LABORATORY  COURSE  IN  PSYCHOLOGY.       [124 

must  be  covered  closely  with  an  opaque  cloth  to  allow  full 
adaptation. 

Aubert,  A,  483  f.,  B,  25  ff. ;  Charpentier,  A,  154  ff. ;  Treitel;  Hering, 
C.  On  b,  Hillebrand. 

AFTER-IMAGES. 

After-images,  Accidental  or  Consecutive  Images.  After- 
images in  which  the  relations  of  light  and  shade  of  the 
original  object  are  preserved  are  called  Positive  After-images. 
Those  in  which  these  relations  are  reversed  (as  in  a  photo- 
graphic negative)  are  called  Negative  After-images.  Posi- 
tive aTter-images  are  of  various  colors,  but  most  important 
to  notice  here  are  those  of  the  color  of  the  object  (like- 
colored),  and  of  the  complementary  color  (opposite-colored). 
Negative  after-images,  so  far  as  observed,  are  always  oppo- 
site-colored. All  after-images,  especially  the  positive,  can 
best  be  observed  in  the  morning  when  the  eyes  are  well 
rested. 

124  Negative  After-images,  a.  Look  steadily  for  a  minute 
at  a  fixed  point  of  the  window,  then  at  a  white  screen  or  an 
evenly  lighted,  imfigured  wall ;  the  dark  parts  of  the  win- 
dow will  now  appear  light  and  the  light  dark. 

b.  Get  a  lasting  after-image  and  look  at  a  corner  of  the 
room,  or  at  a  chair  or  other   object   of  uneven   surface  ; 
notice  how  the  image  seems  to  fit  itself  to  the  surface  upon 
which  it  rests.     After  a  little  practice  it  is  also  possible  at 
will  to  see  the  image  floating  in  the  air  instead  of  lying  on 
the  background. 

c.  Look  steadily  at  a  bright-colored  object  or  some  bits 
of  colored  paper,  then  at  the  screen ;  observe  that  the  colors 
of  the  after-images  are  approximately  complementary  to  the 
colors  of  the  objects  producing  them. 

d.  Negative  After-images  upon  a  Background  faintly  Tinged 
with  the  Stimulating  Color.     Fasten  upon  the  color-mixer  a 


125]  THE  MECHANISM  OF  THE  EYE.  113 

white  disk  upon  which  has  been  painted  a  six  rayed  star  of 
red.  Set  the  disk  in  rapid  rotation,  bring  the  eyes  within 
eight  or  ten  inches  of  the  disk,  and  after  half  a  minute  sud- 
denly withdraw  them  to  thirty  or  forty  inches.  As  the 
head  is  drawn  back  the  complementary  color  will  be  seen 
to  press  in  upon  the  disk  from  all  sides  while  the  red  con- 
tracts. When  the  head  is  again  approached  to  the  disk  the 
red  will  enlarge  and  the  blue-green  disappear.  The  cause 
of  the  rushing  in  of  the  blue  in  the  first  case  is  the  contrac- 
tion of  the  retinal  image,  which  of  course  decreases  in  size 
as  the  head  is  drawn  back,  and  is  thus  brought  upon  parts 
of  the  retina  that  have  been  more  strongly  stimulated. 
When  the  head  approaches  the  disk  the  retinal  image 
enlarges  and  its  outer  portion  lies  on  a  fresh  area.1 

Negative  after-images  are  sometimes  very  lasting,  and  for 
that  reason  are  those  most  frequently  noticed  in  ordinary 
experience.  They  are  phenomena  of  retinal  fatigue  (Helm- 
holtz),  or  of  retinal  restitution  (Hering). 

125.  Positive  After-images.  These  images  are  not  diffi- 
cult to  see,  if  after  a  brief  stimulation  the  eye  is  shielded 
from  further  action  of  lighj;.  Thus,  when  the  gas  is  sud- 
denly turned  off  in  a  dark  room,  the  positive  image  of  the 
flame  and  the  burner  is  very  easily  seen. 

a.  Look  for  an  instant  (one-third  of  a  second)  at  the  win- 
dow, then  close  and  cover  the  eyes.  Notice  that  the  after- 
image is  like  the  window  in  distribution  of  light  and  shade, 
bright  panes  and  dark  bars,  and  at  first  like  it  also  in  color. 
After  some  practice  it  is  also  possible  to  see,  for  a  small 
fraction  of  a  second,  the  positive  after-image  of  almost  any 
bright  object  on  suddenly  turning  the  eyes  from  the  object 
to  some  other  part  of  the  field,  especially  if  the  latter  is 
dark.  The  positive  after-image  is  of  short  duration  and 
less  readily  observed  than  the  negative.  It  has  generally 

1  For  a  still  simpler  experiment,  see  Mind,  Ser.  2,  II.,  1893,  485,  note. 


114       LABORATORY  COURSE  IN  PSYCHOLOGY.      [125 

been  considered  a  phenomenon  of  retinal  inertia,  a  prolon- 
gation of  the  original  retinal  excitation,  and  such  a  prolonga- 
tion does  undoubtedly  exist.  Charpentier  and  Hess,  however, 
in  experiments  with  very  brief  stimulation,  have  found 
a  transient  negative  image  coming  between  the  original 
impression  and  the  ordinary  positive  after-image  observed 
with  longer  stimulation.  The  full  series  would  then  be : 
1.  Prolongation  of  the  original  stimulus ;  2.  First  Negative 
Image ;  3.  Ordinary  Positive  After-image ;  4.  Ordinary 
Negative  After-image. 

b.  Colored  Positive  After-images.     Look  for  an  instant  at 
a  gas  flame  through  a  piece  of  red  glass,  then  close  and 
cover  the  eyes  and  observe  the  red  image ;  repeat  the  exper- 
iment, continuing  the  fixation  of  the  flame  for  half  a  min- 
ute ;  the  resulting  after-image  will  be  bright  as  before  but 
of  the  opposite  color. 

c.  After-images  on  Dark  and  Light  Backgrounds.      Get 
an  after-image  of  the  window  of  not  too  great  intensity,  and 
project  it  alternately  on  a  sheet  of  white  paper  and  the  dark 
field  of  the  closed  and  covered  eyes ;  it  will  be  found  nega- 
tive on  the  white  background?  and  positive  on  the  dark. 
Some  observers  find  a  periodic  reappearance  of  positive 
after-images,  or  an   alternation   of   positive   and  negative 
images,  without  a  change  of  background. 

d.  Sequence  of  Colors.      Get  a  good  after-image  of  the 
window,  and  observe  with  closed  and  covered  eyes  the  play 
of  colors  as  the  image  fades.     Try  several  times  and  observe 
that   the  order  of  succession  is  the  same.      According   to 
Hering,  this  play  of  colors  would  not  take  place  if  the  origi- 
nal stimulus  were  absolutely  colorless. 

On  Exs.  124  and  125,  consult  the  following:  Helmholtz,  A,  480  ff., 
501  ff.,  Fr.  446  (338),  471-500  (357-380);  Wundt,  A,  3te  Aufl.,  I., 
472-476,  4te  Aufl.,  I.,  512  ff.;  Hess;  Charpentier,  B.  See  also  ref- 
erences given  in  Chap.  VI.  for  Successive  Contrast. 


127]  THE  MECHANISM  OF  THE  EYE.  115 

126.  Effect  of  Eye-motions  on  After-images.     Get  a  mod- 
erately strong  after-image  of  the  window  ;  look  at  the  wall 
and  keep  the  eyes  actively  in  motion.     The  image  will  be 
seen  with  difficulty  while  the  eye  is  in  motion ;  when,  how- 
ever, the  eye  is  brought  to  rest,  it  will  soon  appear.     In 
general,  any  visual  stimulus  that  moves  with  the  eye  is  less 
effective  than  one  that  does  not. 

Exner,  A. 

127.  The  Seat  of  the  After-image.     An  after-image  due  to 
stimulation  of  one  eye  may,  under  proper  conditions,  some- 
times seem  to  be  seen  with  the  other.     From  this  it  has 
been  inferred  that  the  seat  of  after-images  is  central,  not 
peripheral ;    that  is',  in  the   visual   centres   of  the   brain, 
higher  or  lower,  not  in  the  retina.     The  following  experi- 
ments show,  however,  that  the  after-image  is  really  seen 
with  the  eye  first  stimulated,  and  so  render  the  hypothesis 
of  a  central  location  unnecessary. 

a.  Look  steadily  for  a  considerable  time  at  a  bit  of  red 
paper  on  a  white  ground,  using  only  one  eye,  say  the  right, 
and  keeping  the  other  ckjsed ;  when  a  strong  after-image 
has   been  secured,  remove  the  paper,  close  the  right   eye, 
open  the  left,  and  again  look  steadily  at  a  fixed  point  on  the 
white  ground ;  after  a  little  the  field  will  darken  and  the 
after-image  will  reappear.     If  the  red  does  not  produce  a 
sufficiently  lasting  image,  substitute  for  it  a  gas  flame  or 
some  other  bright  object. 

b.  That   we   have   really  to  do  with  the   eye   originally 
stimulated  (its  present  dark  field  suppressing  the  light  one  of 
the  other  eye),  appears  from  such  experiments  as  the  fol- 
lowing :    Get   the   after-image  as  before ;    then   open  both 
eyes   and  bring  a  bit  of   cardboard  before  the  eyes  alter- 
nately.    Bringing  it  before  the  left  eye  rather  brightens  the 
image ;  bringing  it  before  the  right  dims  or  abolishes  it. 


116        LABORATORY  COURSE  IN  PSYCHOLOGY.      [128 

The  image  is  thus  chiefly  affected  by  what  affects  the  right 
eye. 

c.  Get  the  after-image  again,  and  close  and  cover  both 
eyes ;  observe  the  color  of  the  after-image,  as  projected  on 
the  dark  field  5  then  open  the  left  eye,  letting  the  right  eye 
remain  closed  and  covered.  The  after-image  will  be  seen, 
not  in  the  color  it  has  when  the  right  eye  is  open  and  the 
image  is  projected  in  the  light  field,  but  in  that  which  it 
has  in  the  dark  field  of  the  closed  eye. 

These  experiments  prove  that  after-images  belong  to  the 
stimulated  half  of  the  visual  apparatus,  but  ihey  do  not 
show  whether  the  images  belong  to  the  retina  of  that  half 
or  to  the  nervous  centres  connected  with  it.  Other  consid- 
erations, such,  for  example,  as  the  fact  that  the  image  fol- 
lows every  motion  of  the  eye,  even  those  that  are  usually 
unconscious,  is  affected  by  pressures  exerted  on  the  eyeball 
and  by  electric  currents  sent  through  it,  together  with  Ex- 
ner's  direct  experiments  on  retinal  and  optic  nerve  stim- 
ulation, support  the  retinal  location,  in  favor  of  which 
current  opinion  is  practically  unanimous.  Some  observers, 
however,  have  been  able  to  get  a  binocular  after-image  of 
a  somewhat  different  character  ;  see  binocular  section  of 
Chap.  VI. 

Delabarre  ;  Exner,  D,  246  ff.  and  E ;  Fick  and  Giirber,  296  ff. 

128.  After-images  of  Motion.  These  after-images  can  be 
secured  from  almost  any  continuously  moving  object.  They 
are  often  unpleasantly  striking  after  looking  at  the  water 
from  the  deck  of  a  vessel  or  at  the  landscape  from  a  car 
window.  In  the  experiments  below,  variations  of  one  of 
the  laboratory  methods  of  producing  them  are  given. 

a.  Fasten  upon  the  rotation  apparatus  a  disk  bearing  a 
large  number  of  equal  black  and  white  sectors;  set  it  in 
slow  rotation  and  gaze  fixedly  at  it.  The  rate  must  not 


128]  THE  MECHANISM  OF  THE  EYE.  117 

be  fast  enough  to  blur  the  outlines  of  the  sectors  very 
much.  After  a  moment  or  two  of  steady  fixation,  bring  it 
suddenly  to  rest  and  observe  its  slow  illusory  backward 
movement. 

b.  Fasten  on  the  apparatus  a  disk  like  that  in  the  accom- 
panying cut,  and  get  an  after-image  as  before,  fixating  the 
centre.  Bring  the  disk 
suddenly  to  rest,  or 
look  away  from  it  to  a 
page  of  print  or  into 
the  face  of  a  bystander 
and  notice  the  apparent 
shrinking  or  swelling, 
reversing  the  previous 
motion  of  the  spiral. 
Illusions  of  increase 
or  decrease  of  distance 
sometimes  accompany 
those  of  motion  with 
this  disk.  Kepeat  the  experiment,  but  this  time  instead  of 
looking  at  some  object,  close  the  eyes  and  turn  them  toward 
the  sky  or  other  source  of  bright  light.  The  apparent  motion 
will  be  observed  again  in  the  red-yellow  field. 

e.  Hold  over  half  of  the  disk  while  in  rotation  a  piece  of 
cardboard,  fixate  the  centre  of  the  disk,  and  get  the  after- 
image. Observe  that  the  after-image  is  limited  to  the 
portion  of  the  retina  stimulated. 

d.  Get  a  monocular  after-image  of  the  spiral,  with  th^ 
right  eye,  for  example.     Then  close  the  right  eye  and  open 
the  left ;  the  after-image  of  motion  will  be  projected  like 
that  of  color  in  Ex.  127. 

e.  Hold  just  above  the  spiral  disk  a  larger  disk  of  paste- 
board, cut  with  a  radial  slot  an  inch  or  two  wide.     When 
the  spiral  is  now  revolved  a  narrow  strip  will  be  seen  in 


118        LABORATORY  COURSE  IN  PSYCHOLOGY.      [128 

which  the  motion  is  in  one  direction  only.  Get  a  strong 
after-image  and  observe  it  with  closed  eyes  as  in  b  above. 
It  will  sometimes  be  possible,  at  least  for  a  short  time,  to 
get  a  reversal  of  the  previous  illusion ;  the  part  of  the  image 
corresponding  to  the  slot  will  appear  to  stand  still  while  the 
adjacent  parts  move,  or  both  will  appear  in  motion  in  op- 
posite directions.  This  experiment  is  apparently  easier 
to  get  with  the  antirrheoscope,  where  the  moving  field  is 
larger.  With  that  instrument  the  effect  mentioned  can  be 
seen  in  the  ordinary  projected  after-image. 

When  a  strong  after-image  is  projected  upon  a  set  of 
straight  lines  at  right  angles  to  the  direction  of  movement, 
some  observers  have  seen  the  lines  more  or  less  distorted  by 
it  (Budde  saw  them  thus  affected  when  the  lines  did  not 
cross,  but  only  entered  the  moving  part  of  the  field)  ;  others 
have  found  the  lines  entirely  unaffected.  It  seems  prob- 
able that  the  breadth  and  distinctness  of  the  lines  have 
something  to  do  with  this  difference  of  results. 

Exner,  who  believes  in  the  retinal  seat  of  color  after- 
images, is  inclined  to  give  a  more  central  location  to  these 
of  motion.  In  his  opinion  such  experiments  as  those  above 
indicate  also  that  our  knowledge  of  such  motions  is  a  sensa- 
tion, not  a  perception. 

After-images  of  motion  have  been  explained  by  actual, 
though  unconscious,  movements  of  the  eyes,  like  the  ap- 
parent movements  of  objects  in  dizziness.  This  is  certainly 
incorrect ;  for  in  b  it  would  seem  necessary  that  the  eyes 
should  move  in  all  directions  at  once,  and  c  shows  that  the 
effect  is  limited  to  a  portion  of  the  field,  which  would  be 
impossible  if  it  were  due  to  actual  eye  motions.  The  same 
was  demonstrated  by  Dvorak  by  means  of  a  disk  with  three 
concentric  spirals,  the  inner  and  outer  ones  being  drawn 
in  the  same  way,  (right-handed  spirals,  for  example),  while 
that  between  was  drawn  in  the  reverse  direction.  How  far 


128]  THE  MECHANISM  OF  THE  EYE.  119 

some  psychical  representation  of  ocular  motions  co-operates 
in  the  illusion  would  be  hard  to  say. 

Helmholtz,  A,  Fr.  766-769  (603-605);  Bowditch  and  Hall;  Mach, 
A,  59-61  (see  also  61-65  for  yet  another  kind  of  after-image),  and  13, 
65-67;  Exner,  B  and  C,  440  ff.;  Dvorak;  Budde;  von  Fleischl; 
Heuse;  Zehfuss. 

MOVEMENTS  OF  THE  EYES. 

The  eye  is  a  moving  as  well  as  a  seeing  member ;  and  its 
motor  functions  are  of  great  importance  for  psychology,  es- 
pecially for  the  theory  of  the  visual  perception  of  space. 
The  experiences  of  the  eye  in  motion  have  a  controlling 
influence  upon  its  perceptions  even  when  at  rest,  as  will 
appear  in  some  of  the  experiments  of  Chap.  VII. 

All  motions  of  the  eye  may  be  conceived  as  rotations  of 
greater  or  less  extent  about  one  or  more  of  three  axes :  a 
sagittal  axis,  corresponding  nearly  with  the  line  of  sight ; 
a  frontal  axis,  extending  horizontally  from  right  to  left ;  and 
a  vertical  axis.  Theoretically  all  these  intersect  at  right 
angles  in  the  Centre  of  Rotation  of  the  eye.  As  a  land- 
mark from  which  to  measure  eye-movements,  that  position 
(approximately)  is  taken  which  the  eyes  assume  when  the 
head  and  body  are  erect  and  the  eyes  are  directed  forward 
to  a  distant  horizon.  This  is  known  as  the  Primary  Posi- 
tion of  the  eyes  (or  the  lines  of  sight) ;  any  other  is  a 
Secondary  Position.  The  point  on  which  the  eyes  are  fixed 
when  in  the  primary  position  is  the  Primary  Fixation 
Point,  or  Principal  Point  of  Regard.  The  Field  of  Vision 
is  the  extent  of  space  that  can  be  seen  with  the  eye  at 
rest.  The  Field  of  Regard  is  the  extent  of  space  that  can 
be  seen  when  the  eyes  are  moved.  In  the  following  experi- 
ments the  word  Rotation,  except  in  the  expression  "  centre 
of  rotation,"  is  reserved  for  turnings  about  the  sagittal 
axis. 


120        LABORATORY  COURSE  IN  PSYCHOLOGY.      [131 

129.  Keflex  Movements  of  the  Eye.     Of  the  first  impois 
tance  among  eye  movements  is  the  constant  reflex  tendency 
of  the  eye  to  move  in  such  a  way  as  to  bring  any  bright 
image  lying  on  a  peripheral  part  of  the  retina,  or  any  to 
which  attention  is  directed,  into  the  area  of  clearest  vision. 
Many   evidences    of  this   tendency  will   be  found   in  the 
ordinary  course  of  vision.     By  way  of  experiment,  try  to 
study  attentively  a  musca  volitans  or  a  negative  after-image 
that  is  just  to  one  side  of  the  direct  line  of  sight.     The 
apparent  motion  of  the  object  measures  the  energy  of  the 
reflex. 

130.  Associated  Movements  of  the  Eyes.     The  two  eyes 
form  a  single  visual  instrument ;   and  even  when  one  eye 
is  closed,  it  follows  to  a  considerable  degree  the  movements 
of  its  open  companion.     Movements  upward  or  downward 
in  normal  vision  are  always  performed  simultaneously  by 
the  two  eyes. 

a.  Close  one  eye,  and,  resting  the  finger-tip  lightly  on  the 
lid,  feel  the  motions  of  that  eye  as  the   other  looks  from 
point  to  point  of  the  field  of  regard. 

b.  Get  a  monocular  after-image,  as  in  Ex.  127,  and  when 
it  seems  visible   to    the  open   eye,   notice  that   it   accom- 
panies the  fixation  point  of  that  eye  as  it  moves  from  point 
to  point  of  the  field  of  regard. 

Aubert,  A,  651  ff.;  Bering,  A,  519  ff. 

131.  Motions  of  the  Eyes  when  the  Lines  of  Sight  are 
Parallel.     The   movements  here   considered  are  somewhat 
simplified  for  easier  exposition. 

a.  Donders's  Law  ;  the  Law  of  Constant  Orientation 
(Helmholtz)  ;  the  Law  of  Like  Position  with  Like  Direction 
(Hering).  It  is  evident  that  when  the  eye  is  fixed  upon  some 
point  of  its  field,  e.g.,  ten  degrees  upward  and  fifteen  degrees 
to  the  right  of  the  primary  position,  it  is  not  thereby  fixed 


131]  THE  MECHANISM  OF  THE  EYE.  121 

as  regards  its  sagittal  axis,  but  might  conceivably  assume 
an  indefinite  number  of  positions  by  different  degrees  of  rota- 
tion about  that  axis.  It  might  also,  if  not  entirely  free  in 
its  rotation,  rotate  now  through  one  angle  and  now  through 
another,  depending  on  the  direction  in  which  the  line  of 
sight  had  moved  to  reach  the  position  in  which  it  is  then 
found.  As  a  matter  of  fact,  however,  it  does  not  assume 
an  indefinite  number  of  positions,  but  one  and  only  one,  no 
matter  by  what  movements  the  line  of  sight  has  come  to 
that  point.  This  is  Donders's  Law;  and  the  fact  that  it 
expresses  is  of  importance  for  sure  and  easy  recognition  of 
directions  in  the  field  of  regard,  and  for  deciding  whether 
or  not  objects  in  the  field  have  moved  when  the  eye  itself 
has  been  moved.  The  correctness  of  this  law  is  easy  to 
demonstrate. 

Cut  in  a  sheet  of  black  cardboard  two  slits  an  eighth  of 
an  inch  wide  and  four  or  five  inches  long,  crossing  at  right 
angles.  Set  the  cardboard  in  the  window  or  before  some 
other  brightly  lighted  surface.  Arrange  a  head-rest  at  a 
considerable  distance,  and  when  the  head  is  in  position,  get 
a  strong  after-image  of  the  cross,  fixating  its  middle  point. 
Then,  without  moving  the  head,  turn  the  eyes  to  different 
parts  of  the  walls  and  ceiling.  The  image  will  suffer 
various  distortions  from  the  different  surfaces  upon  which  it 
is  projected,  but  each  time  the  eye  returns  to  the  same  point 
the  image  will  lie  as  before.  If  the  wall  does  not  offer  fig- 
ures by  which  this  can  be  determined,  have  an  assistant 
mark  the  position  of  the  image  upon  it.  The  after-image  is 
of  course  fixed  on  the  retina  and  can  move  only  as  the  eye 
moves. 

b.  Listing's  Law.  This  law  goes  beyond  Donders's  Law, 
and  asserts  that  the  position  is  not  only  fixed,  but  that  in 
movements  from  the  primary  position  there  is  no  rotation 
at  all  about  the  sagittal  axis.  In  other  Avords,  the  final  posi- 


122        LABORATORY  COURSE  IN  PSYCHOLOGY.      [131 

tion  is  such,  as  the  eye  would  assume  if  it  were  moved  from 
its  primary  position  to  the  position  in  question  by  turning 
about  a  fixed  axis  standing  perpendicular  at  the  centre  of 
rotation  to  both  the  primary  and  the  new  position  of  the 
line  of  sight.  To  show  this  requires  a  little  more  care  than 
the  last  experiment. 

The  observer  must  be  placed  at  a  distance  of  twenty-five 
or  thirty  feet  from  an  extensive  wall  space,  with  a  suitable 
head-rest  as  before.  The  lines  of  sight  are,  of  course,  not 
strictly  parallel  at  this  distance,  but  the  difference  may  be 
neglected.  On  the  wall  stretch  dark-colored  strings  as  indi- 
cated in  the  accompanying  diagram.  The  cross  at  the  lower 
right  hand  corner  should  be  approximately  in  the  primary 
position  for  the  observer.  The  longer  vertical  and  horizontal 
strings  should  be  twelve  or  fifteen  feet  long,  the  inclined 
one  eighteen  or  twenty  feet.  The  angle  that  the  last  makes 
with  the  others  is  not  important  so  long  as  it  is  not  too 

small  with  either.  Fix- 
ation points  of  black 
cardboard  or  some  other 
conspicuous  substance 
should  be  affixed  as  indi- 
cated by  the  little  circles. 
The  cross  in  the  corner 
may  be  made  by  pasting 
strips  of  bright-colored 
paper  half  an  inch  wide 
and  a  foot  long  on  a 
disk  of  white  car  d- 
board,  or  (better  still)  it 
may  be  made  by  the  line 
of  junction  of  four  colored  set-tors,  two  red  and  two  blue,  for 
example.  The  disk  in  either  case  must  be  so  arranged  that 
it  can  be  turned  about  its  centre  and  one  of  its  diameters 


131]  THE  MECHANISM  OF  THE  EYE.  123 

be  made  to  coincide  with  the  oblique  string.  When  all  has 
been  arranged  make  the  following  tests  :  — 

Exact  determination  of  the  primary  position.  For  most 
observers  this  is  somewhat  depressed  below  the  horizontal 
position.  Let  the  observer  fixate  the  centre  of  the  disk 
till  he  has  secured  a  strong  and  clear-cut  after-image  of  it 
and  then  turn  his  eyes,  taking  care  not  to  move  his  head, 
to  the  fixation  marks  on  the  horizontal  and  vertical  strings. 
If  the  corresponding  lines  of  the  after-image  coincide  with 
the  strings,  the  head  is  in  the  required  position.  If  not,  the 
head  must  be  moved  a  little  to  right  or  left  if  the  error 
is  with  the  vertical  bar,  and  up  or  down  if  with  the  hori- 
zontal. The  primary  position  differs  a  little  from  observer 
to  observer,  and  even  with  the  same  observer  at  different 
times. 

Having  found  the  primary  position,  have  an  assistant 
turn  the  cross  disk  so  that  one  of  its  diameters  coincides 
with  the  oblique  string.  Get  a  clear  after-image  of  it,  and 
look  at  the  fixation  point  on  that  string.  Again  the  bar  of 
the  cross  will  lie  exactly  upon  the  string,  thus  showing  that 
no  rotation  of  the  eye  about  the  line  of  sight  has  taken 
place.  The  same  would  be  true  for  any  other  direction  of 
motion  from  the  primary  position,  provided  the  movement 
were  not  of  extreme  extent.  There  is  then  a  set  of  lines, 
radiating  from  the  primary  fixation  point,  along  which  the 
eye  can  move,  so  as  to  bring  all  parts  of  the  same  line  suc- 
cessively on  the  same  part  of  the  retina.  Direct  examina- 
tion of  such  a  line  and  comparison  of  its  parts  is  easy. 

Restore  the  cross  disk  to  its  first  position,  incline  the 
head  forward  or  backward,  or  turn  it  to  right  or  left  before 
getting  the  after-image  (thus  bringing  the  eye  into  a  sec- 
ondary position),  and  repeat  the  experiments  just  made. 
Notice  that  the  bars  do  not  now  coincide  with  the  strings, 
showing  that  the  eyes  have  suffered  a  certain  amount  of 


124        LABORATORY  COURSE  IN  PSYCHOLOGY.     [132 

rotation.  Such  a  rotation  appears  for  all  secondary  posi- 
tions (except  when  the  fixation  point  both  at  starting  and 
ending  lies  in  a  straight  line  passing  through  the  primary 
fixation  point),  but  the  extent  of  it  is  small  in  the  ordinary 
movements  of  the  eyes,  and  extreme  movements  are  usually 
avoided  by  simultaneous  movements  of  the  head. 

With  the  cross  on  the  disk  vertical  as  in  the  cut,  get  an 
after-image  and  fixate  the  mark  on  the  oblique  string.  In- 
stead of  being  rectangular  as  before,  the  after-image  cross 
now  appears  somewhat  distorted,  like  an  oblique  X.  The 
after-image  on  the  retina  of  course  remains  rectangular. 
The  distortion  of  the  image  on  the  wall  is  the  result  of  the 
interpretation  now  placed  upon  it  by  the  mind.  The  short 
string  cross  at  the  same  centre  is  known  to  be  rectangular, 
and  if  the  after-image  cross  fails  to  agree  with  it,  the  only 
harmonization  of  the  two  is  that  the  latter  is  not  really 
rectangular.  Oblique  crosses  in  such  a  position  in  previous 
experience  have  given  rise  to  rectangular  retinal  images 
so  often  that  this  interpretation  is  immediate,  and  seems 
wholly  a  matter  of  sensation. 

For  a  fuller  account  of  Listing's  Law  see  Appendix  I. 

Of.  Helmholtz,  A,  Fr.  601-609  (462-469),  621  (479)  ff.,  702(548)  ff.; 
Aubert,  A,  653  ff. ;  Wundt,  A,  3te.  Aufl.,  II.,  94  ff.  ;  Hering,  B,  248  ff. ; 
Le  Conte,  164-177. 

132.  Actual  Movements  of  the  Eyes.  Wundt-Lamansky 
Law.  Rapid  motions  of  the  eyes  when  they  move  freely 
and  do  not  follow  strongly  marked  lines  in  the  field  of  re- 
gard, are  not  executed  exactly  according  to  Listing's  Law, 
though  that  gives  correctly  the  end  positions  reached.  The 
axis  about  which  the  eye  turns  is  not  always  constant,  and 
the  paths  of  the  fixation  point  as  it  moves  in  the  field  of  re- 
gard are  therefore  not  all  straight.  This  is  easy  to  observe 
as  follows.  In  a  dark  room  turn  down  the  gas  till  it  burns 
in  a  very  small  flame.  Then  using  this  as  a  distant  point 


133]  THE  MECHANISM  OF  THE  EYE.  125 

of  departure  in  the  primary  position,  look  suddenly  from  it 
to  other  points  of  fixation  in  various  directions  about  it,  and 
notice  the  shape  of  the  long  positive  after-images  that  result 
from  the  motion  of  the  image  of  the  flame  over  the  retina. 
These  will  probably  have  the  shape  of  the  radii  in  the  left 
hand  figure  below,  the  vertical  and  horizontal  being  nearly 
straight,  and  the  oblique  curved.  These,  however,  do  not 
show  immediately  the  track  of  the  fixation  point.  The 
newest  part  of  the  after-image  is  that  next  the  light,  the 
oldest  part  is  that  next  the  fixation  point  —  at  a  in  the 
diagram.  If  the  points  of  the  after-image  curve  are  now 
interpreted  in  the  order  of  time  (taking  the  oblique  curve  to 


the  right  and  upward,  for  example),  it  appears  that  the  eye 
at  first  moved  rather  rapidly  toward  the  right,  but  rather 
slowly  upward,  while  at  last  it  moved  rather  slowly  toward 
the  right  and  rapidly  upward.  Plotting  a  curve  in  accord- 
ant with  this  interpretation,  we  get  that  given  in  B,  which 
shows  the  true  track  of  the  fixation  point.  By  similar  plot- 
ting the  other  tracks  may  be  found. 

It  is  said  that  for  some  eyes  the  after-images,  though 
curved,  do  not  coincide  with  those  figured  in  A. 

Wundt,  B,  139  ff.,  201-202;  Bering,  A,  450-451;  Lamansky. 

133.  Convergent  Movements  of  the  Eyes.  The  laws  of 
Ex.  131  do  not  hold  for  convergent  motions  of  the  eyes. 


126        LABORATORY  COURSE  IN  PSYCHOLOGY.     [133 

When  the  lines  of  sight  converge  in  the  primary  position, 
both  eyes  rotate  outward  ;  as  the  lines  of  sight  are  elevated, 
the  convergence  remaining  the  same,  the  outward  rotation 
increases  ;  as  they  are  depressed,  the  rotation  diminishes 
and  finally  becomes  zero.  On  a  sheet  of  cardboard  draw  a 
series  of  equidistant  parallel  vertical  lines  one  or  two  inches 
apart  and  eight  or  ten  inches  long,  drawing  the  left  half  of 
the  group  in  black  ink,  the  right  half  in  red.  Cross  both 
sets  midway  from  top  to  bottom  by  a  horizontal  line,  red  in 
the  red  set,  and  black  in  the  black  set.  Fasten  the  card- 
board flat  upon  a  vertical  support,  and  arrange  the  head  rest 
in  front  of  it.  The  horizontal  line  of  the  diagram  should 
be  on  a  level  with  the  eyes. 

a.  If  the  operator  is  unable  to  control  the  degree  of  con- 
vergence voluntarily,  he  should  fasten  a  bit  of  wire  vertically 
between  his  eyes  and  the  diagram  in  such  a  way  that  it  can 
be  moved  to  and  from  the  eyes.     If  he  is  able  to  control  the 
convergence  voluntarily,  the  wire  is  unnecessary.     Bring 
the  head  into  position  and  converge  the  eyes,  giving  atten- 
tion to  the  diagram.     It  will  be  seen  that  the  red  and  black 
lines  are  not  quite  parallel  (or  do  not  quite  coincide),  and 
that  they  are  less  nearly  so  as  the  convergence  is  increased. 
The  red  lines  (seen  by  the  left  eye)  seem  to  incline  a  little 
toward  the  right,  and  the  black  lines  (seen  by  the  right  eye) 
toward  the  left.     When  the  convergence  is  great,  the  hori- 
zontal lines  also  will  show  the  rotation.      This  apparent 
rotation  of  the  lines  is  not,  as  in  the  case  of  the  after-image, 
a  sign  that  the  corresponding  eye  has  rotated  in  the  same 
way,  but  that  it  has  rotated  in  the  opposite  way. 

b.  Repeat   this  with   the  head   much  inclined   forward 
(the  equivalent  of  elevating  the  eyes)  and  with  it  thrown 
far  back  (equivalent  of  depressing  the  eyes),  taking  care 
that  the  same  degree  of  convergence  is  maintained.     In  the 
first  case  the  apparent  rotation  of  the  lines  is  increased,  and 


134]  THE  MECHANISM  OF  THE  EYE.  127 

in  the  second  decreased  to  zero,  or  even  transformed  into 
rotation  in  the  opposite  direction. 

Helmholtz,  A,  Fr.  609-610  (469-470);  Le  Conte,  177-191;  Bering, 
A,  496  ff.;  Aubert,  A,  658  ff. 

134.  Involuntary  Movements  of  the  Eyes.  Lay  a  small 
scrap  of  red  paper  on  a  large  piece  of  blue.  Fixate  some 
point  on  the  edge  of  the  red.  After  a  few  seconds  of  steady 
fixation,  the  color  near  the  line  of  separation  will  be  seen 
to  brighten,  now  in  the  red  and  now  in  the  blue,  thus  be- 
traying the  small  unintentional  movements  of  the  eyes. 

Helmholtz,  A,  539,  Fr.  511  (389). 


BIBLIOGRAPHY. 

ATJBEET:  A.  Grundziige  der  physiologischen  Optik,  Leipzig,  1876. 
This  work,  though  obtainable  separately,  forms  a  part  of  the 
second  volume  of  v.  Graefe  and  Saemisch's  Handbuch  der 
gesammten  Augenheilktmde.  It  contains  in  its  three  hundred 
pages  a  very  large  amount  of  matter  stated  with  great  brevity 
and  clearness,  and  is  in  every  way  excellent. 
B.  Physiologic  der  Netzhaut,  Breslau,  1865. 

YON  BEZOLD:  Ueber  Zerstreuungsbilder  auf  der  Netzhaut,  v. 
Graefe's  Archiv,  XIV.,  1868,  ii.,  1-29. 

BOWDITCH  AND  HALL:  Optical  Illusions  of  Motion,  Journal  of 
Physiology,  III.,  1881-82,  297-307. 

BUDDE:  Ueber  metakinetische  Scheinbewegungen  und  iib.er  die 
Wahrnehmung  der  Bewegung,  Du  Bois-Reymond'1 s  Archiv, 
1884,  127-152. 

CHABPENTIER:  A.   La  lumiere  et  les  couleurs,  Paris,  1888. 

B.  Reaction  oscillatoire  de  la  retine  sous  1'  influence  des  excitations 
lumineuses,  Archives  de  Physiologic,  Ser.  5,  IV.,  1892,  541-553, 
629-639.  Essential  points  of  the  same,  Comptes  rendus,  CXIII., 
1891,  147-150,  217-219.  See  also  Nature,  XL VIII.,  1893,  380. 

DELABAKEE:  On  the  Seat  of  Optical  After-images,  American  Jour- 
nal of  Psychology,  II.,  1888-89,  326-328. 


128        LABORATORY  COURSE  IN  PSYCHOLOGY. 

DVOKAK:  Versuche  iiber  die  Nachbikler  von  Reizveranderungen, 
Sitz.-ber.  d.  k.  Akademie  d.  Wiss.  i.  Wien,  math.-nat.  Classe, 
LXI.,  1870,  Abth.,  ii.  257-262. 

EXNER :  A.  Das  Yerscliwinden  der  Nachbilder  bei  Augenbewegung- 
en,  Zeitschrift  fur  Psychologic,  L,  1890,  47-51. 

B.  Einige   Beobachtungen   iiber  Bewegungsnachbilder,  Central- 
Uattfur  Physiologic,  I.,  1887,  No.  6,  135-140. 

C.  Ueber  optische   Bewegungsempfindungen,  Biologisches  Cen- 
tralblatt,  VIII.,  1888,  No.  14,  437-448. 

D.  Ueber  die  Functionsweise  der  Netzhautperipherie  und  den 
Sitz  der  Nachbilder,  v.  Graefe's  Archiv,  XXXII.,  1886,  i.,  233- 
252. 

E.  Ueber  den  Sitz  der  Nachbilder  im  Centralnervensystem,  Rep. 
der  Physik,  XX.,  Protokoll  d.  chem.  phys.  Ges.  in  Wien,  18 
Marz,  1884. 

F.  Ueber  einige  neue  subjective  Gesichtserscheinungen,  Pfluger^s 
Archiv,  L,  1868,  375-394. 

TICK,   A.:    A.   Dioptrik  und  Nebenapparate  des  Auges,  Hermann's 

Handbuch  der  Physiologic,  III.,  i.  3-138. 
B.   Die  Lehre  von  der  Lichtempfindung.  ibid.,  139-234. 

FICK,  A.  E.,  AND  GURBER:  Ueber  Erholung  der  Netzhaut,  v.  Graefe's 
Archiv,  XXXVI.,  1890,  ii.,  245-301.  See  also  Hering's  critique, 
Fick's  reply,  Hering's  rejoinder,  and  Fick's  second  reply. 
ibid.,  XXXVII.  and  XXXVIII. 

VON  FLEISCHL:  Physiologisch-optische  Notizen  (2te  Mittheilung), 
Sitz.-ber.  d.  k.  Akademie  d.  Wiss.  i.  Wien,  math.-nat.  Classe, 
LXXXVL,  1882,  Abth.  iii.,  8-25. 

HELMHOLTZ:  A.   Handbuch  der  physiologischen  Optik,  2te.  Aufl., 

Hamburg  und  Leipzig,  1886-1892. 

Of  this  second  edition  of  Helmholtz's  work  but  seven  parts  have 
so  far  appeared.  The  latest  complete  edition  is  the  French 
translation  by  Javal  and  Klein  (Optique  physiologique,  Paris, 
1867).  The  references  following  the  experiments  are  given  when 
possible  for  both  the  second  German  edition  and  the  transla- 
tion. The  figures  in  parentheses  following  those  for  the  trans- 
lation are  the  pages  of  the  first  German  edition  taken  from  the 
double  paging  of  the  French  version.  Having  been  taken  thus 
at  second  hand,  they  may  sometimes  be  in  error  by  a  page  or 


THE  MECHANISM  OF  THE  EYE.  129 

two,  but  it  seemed  better  to  run  that  risk  than  to  omit  them 

altogether.     It  is  hardly  necessary  to  add  that  this  work  is 

above  all  others  the  masterpiece  of  physiological  and  psycho- 
logical optics. 
B.   Die   Stoning  der  Wahrnehmung  kleinster  Helligkeitsunter- 

schiede   durch  das   Eigenlicht   der   Netzhaut,    Zeitschrift  fur 

Psychologic,  I.,  1890,  5-17. 
HEEING  :  A.   Der   Raumsinn    und    die    Bewegungen    des    Auges, 

Hermann's  Handbuch  der  Physiologic,  III.,  Th.  i.,  343-601. 
B.    Beitriige  zur  Physiologic,  Leipzig,  1861-64. 
(7.    Ueber  den  Einfluss  der  Macula  lutea  auf  spectrale  Farben- 

gleichungen,  Pfluger's  Archiv,  LIV.,  1893,  277-318. 
HESS:  Untersuchungen  iiber  die  nach  kurzdauernder  Reizung  des 

Sehorgans  auftretenden  Nachbilder,  Pfliiger's  Archiv,  XLIX., 

1891,  190-208. 
HEUSE  :  Zwei  kleinere  Mittheilungen  aus  dem  Gebiete  der  physio- 

logischen  Optik,  v.  Graefe's  Archiv,  XXXIV.,  1888,  ii.,  127- 

134. 
HILLEBRAND:  Ueber  die   spccifische   Helligkeit    der  Farben   (mit 

Vorbemerkungen  von  E.  Hering).    Sitz.-ber.  d.  k.  Akademie  d. 

Wiss.i.  Wien,  math.-nat.  Classe,  XCVIII.,  1889,  Abth.  iii.,  70- 

120. 
LAMANSKY:   Bestimmung  der  Winkelgeschwindigkeit   der    Blick- 

bewegung,  respective  Augenbewegung,  Pfliiger'>s  Archiv,  II., 

1869,  418-422. 
LAQUEUE:    Ueber    pseudentoptische    Gesichtswahrnehmungen,    v. 

Graefe's  Archiv,  XXXVI.,  1890,  i.,  62-82.     Contains  historical 

references. 

LE  CONTE  :  Sight,  New  York,  1881. 
MACH:  A.  and  B.,  works  cited  with  same  letters  in  bibliography 

of  Chap.  II. 
MAXWELL:    On  Color-vision  at  Different    Points   of   the   Eetina, 

Report  of  the  British  Association,  1870;  or  Maxwell's  Scientific 

Papers,  Cambridge,  1890,  Vol.  II.,  230. 
ROOD  :    On  a  probable  means  of  rendering  visible  the  Circulation 

in  the  Eye,  American  Journal  of  Science,  2d   Ser.,  XXX., 

1860,  264.     Additional  observations  on  the  Circulation  in  the 

Eye,  ibid.,  385. 


130        LABORATORY  COURSE  IN  PSYCHOLOGY. 

SACHS:  Ueber  die  specifische  Lichtabsorption  des  gelben  Fleckes 
der  Netzhaut,  Pfluger's  Archiv,  L.,  1891,  574-586. 

SCHWAKZ:  Ueber  die  Wirkung  des  constanten  Stroms  auf  das 
normale  Auge,  Archiv  fur  Psychiatric,  XXI.,  1890,  588-617. 

TBEITEL:  Ueber  das  Verhalten  der  normalen  Adaptation,  v.Graefe's 
Archiv,  XXXIII.,  1887,  ii.,  73-112. 

TSCHEENING:  Beitrage  zur  Dioptrik  des  Auges,  Zeitschrift  fur 
Psychologic,  III.,  1892,  429^92. 

TUMLTRZ:  Ueber  ein  einf aches  Verfahren,  die  Farbenzerstreuung 
des  Auges  direkt  zu  sehen,  Pfluger's  Archiv,  XL.',  1887,  394. 

UHTHOFF:  Ueber  die  kleinsten  wahrnehmbaren  Gesichtswinkel  in 
den  verschiedenen  Teilen  des  Spektrums,  Zeitschrift  fiir  Psy- 
chologic, I.,  1890,  155-160.  Contains  bibliographical  notices 
on  minimum  visibile. 

WOLF  :  Ueber  die  Farbenzerstreuung  im  Auge,  Wiedemanris  An- 
nalen,  XXXIII.,  1888,  548-554. 

WUNDT  :    A.  Work  cited  in  bibliography  of  Chapter  I. 

B.  Beitrage  zur  Theorie  der  Sinneswahrnehmung,  Leipzig,  1862. 

ZEHFUSS:  Ueber  Bewegungsnachbilder,  Wiedemanri's  Annalen,  IX., 

1880,  672-676. 
The  works  of  Helmholtz  and  Aubert  mentioned  above  contain 

full  bibliographies  for  the  earlier  literature  of  all  the  subjects  con- 
sidered in  this  and  the  next  two  chapters. 


SENSATIONS   OF  LIGHT  AND  COLOR.  131 


CHAPTER   VI. 
Sensations  of  Light  and  Color. 

THE  aim  of  the  following  experiments  is  not  to  settle 
conflicting  color  theories,  but  rather  to  present  the  most  im- 
portant experimental  facts  which  all  color  theories  must 
take  into  account.1  Authoritative  statements  of  theories 
may  be  found  as  follows :  Young-Helmholtz  theory  ;  Helm- 
holtz,  A,  344-350,  Fr.  380-387,  424-425,  484  (290-294, 
320-321,  367)  ;  B,  249-256.  Bering's  theory  ;  Hering,  A, 
70-141 ;  M}  76-79.  Hering  has  not  yet  made  a  general  state- 
ment of  his  theory  in  its  later  developments,  and  his  present 
views  must  be  gathered  in  more  or  less  fragmentary  con- 
dition from  his  numerous  special  articles.  The  theories  of 
Helmholtz  and  Hering  are  the  most  prominent  of  current 
theories ;  and  something  on  them,  especially  on  the  first, 
will  be  found  in  the  physiologies  generally,  and  in  some 
works  on  color  in  the  arts.  Of  other  theories  there  are  a 
considerable  number ;  see,  for  some  of  them,  von  Kries ; 
Wundt,  A,  and  B ;  Bonders,  A  and  E\  Christine  Ladd 
Franklin,  A  and  B ;  Ebbinghaus,  A. 

Most  color  theories  attempt  to  simplify  the  multiplicity 
of  ordinary  color  sensations  by  considering  them  as  com- 
pounds of  a  small  number  of  simple  or  primary  sensations. 
The  number  of  primary  colors  is  different  in  different 
theories ;  red,  green,  and  violet  (or  blue)  are  selected  by 


1  For  concise  statements  of  these  facts,  see  Wundt,  A,  3te  Aufl.,  I.,  487,  501, 
4te  Aufl.,  I.,  529;  and  Christine  Ladd  Franklin,  A. 


132         LABORATORY  COURSE  IN  PSYCHOLOGY. 

the  supporters  of  the  Young-Helmholtz  theory  ;  red,  green, 
yellow,  and  blue  by  Hering,  Mach,  and  others ;  while 
Wundt  is  indisposed  to  make  any  particular  colors  more 
original  for  sensation  than  the  rest.  The  selection  has 
generally  been  dictated  by  considerations  of  physics,  or  the 
results  of  introspective  analysis  of  the  sensations ;  but 
efforts  have  lately  been  made  to  settle  the  question  by 
careful  examination  of  the  color-blind,  and  by  calcula- 
tions based  upon  careful  experiments.  On  the  first,  see 
the  literature  on  color-blindness  below ;  on  the  second,  see 
Helmholtz,  A,  456  ff.,  Z>,  and  Konig  und  Dieterici,  A. 
White  is  unquestionably  a  sensation,  and  Helmholtz  and 
Hering  agree  in  holding  the  same  with  reference  to  black ; 
though  Fick  and  some  others  disagree,  regarding  it  rather 
as  the  absence  of  sensation. 

A  given  color  sensation  may  be  changed  in  three  ways : 
in  color-tone,  in  saturation,  and  in  intensity,  or,  to  use 
Maxwell's  terms,  in  hue,  tint}  and  shade.  Changes  in  color- 
tone  are  such  as  are  experienced  when  the  eye  runs  through 
the  successive  colors  of  the  spectrum.  Changes  in  satura- 
tion are  such  as  are  produced  by  the  addition  or  subtrac- 
tion of  white ;  when  much  white  light  is  added,  the  color 
is  a  little  saturated.  Changes  in  intensity  are  changes  in 
the  brightness  of  the  color.  Changes  in  saturation  and  in 
intensity,  if  excessive,  involve  some  change  of  color-tone 
also.  Hering's  theory  does  not  admit  changes  in  the  inten- 
sity of  light  and  color  sensations  in  any  ordinary  sense  of 
the  word.  Colors  that  by  others  are  said  to  be  of  low  in- 
tensity are  regarded  by  Hering  and  his  school  as  mixed 
with  a  large  proportion  of  black ;  similarly  those  of  high 
intensity  are  mixed  with  much  white.  In  Hering's  theory 
the  possible  changes  are  then  reduced  to  two ;  changes  in 
color-tone  and  in  saturation,  the  latter  including  admixtures 
of  both  white  and  black  (Hillebrand;  Hering,  A}  51  ff.). 


135]  SENSATIONS   OF  LIGHT  AND   COLOR.  133 

In  this  group  of  experiments  it  has  seemed  best  to  follow 
the  better  known  terminology,  though  Hering's  conception 
of  the  matter  ought  not  to  be  disregarded. 

LIGHT  AND  COLOR  IN  GENERAL. 

135.  Color-Blindness,  Holmgren's  Method,  a.  Spread 
the  worsteds  on  a  white  cloth  in  good  daylight.  Pick  out  a 
pale  green  (i.  e.,  a  little  saturated  green)  that  leans  neither 
toward  the  blue  nor  the  yellow  ;  lay  it  by  itself  and  require 
the  person  under  examination  to  pick  out  and  lay  beside  it 
all  other  skeins  that  are  colored  like  it,  not  confining  him- 
self, however,  to  exact  matches,  but  taking  somewhat  darker 
and  lighter  shades  also,  so  long  as  the  difference  is  only  in 
brightness  and  not  in  color-tone.  Do  not  tell  him  to  pick 
out  "the  greens"  nor  require  him  to  use  or  understand 
color  words  in  any  way ;  simply  require  the  sorting.  If 
he  makes  errors,  putting  grays,  light  browns,  salmons,  or 
straws l  with  the  green,  he  is  color-blind ;  if  he  hesitates 
over  the  erroneous  colors  and  has  considerable  difficulty,  his 
color-vision  is  probably  defective,  but  in  a  less  degree. 

b.  If  the  experimentee  makes  errors,  try  him  further  to 
discover  whether  he  is  "  red-blind "  or  "  green-blind  "  by 
asking  him  to  select  the  colors,  including  darker  and  lighter 
shades,  that  resemble  a  purple  (magenta)  skein.  If  he  is 
red-blind,  he  will  err  by  selecting  blues  or  violets,  or  both ; 
if  he  is  green-blind,  he  will  select  green  or  gray,  or  both, 
and  if  he  chooses  any  blues  and  violets,  they  will  be  the 
brightest  shades.  If  he  makes  no  errors  in  this  case,  after 
having  made  them  in  the  previous  case,  his  color-blindness  is" 
incomplete.  Violet-blindness  is  rare.  See  also  Ex.  141  b. 

Complete  certainty  in  the  use   of  even   such   a   simple 

l  It  is  difficult  to  give  the  tints  accurately  in  words.  The  experimenter  should 
consult  the  colored  charts  given  in  the  works  of  Jeffries  mentioned  in  the  bibli- 
ography, and  in  Rayleigh,  B. 


134         LABORATORY   COURSE  IN  PSYCHOLOGY.       [136 

method  as  this  is  not  to  be  expected  without  a  full  study  of 
it  and  experience  in  its  application.  Helmholtz,  Hering, 
Konig,  Kirsehmann,  and  others  give  exact  methods  for 
determining  the  particular  colors  that  are  lacking  in  the 
vision  of  the  color-blind. 

On  color-blindness  and  methods  of  testing  for  it,  see  Helmholtz, 
A,  357-372,  456-462;  Fr.  388-399,  (294-300,  847-848);  Holmgren; 
Jeffries,  A  and  5;  Rayleigh,  A  and  B;  Hering,  £f,  7,  JV,  Hess,  B; 
Abney,  -4;  Abney  and  Festing;  Konig,  B  and  C;  Brodhun,  A  and 
JB;  Konig  und  Brodhun;  Konig  und  Dieterici,  A\  Schuster;  Preyer; 
Bonders,  C;  Kirschmann,  A ;  Pole. 

136.  Vision  with  Peripheral  Portions  of  the  Retina: 
Perception  of  Light.  A  very  faint  light  often  appears 
brighter  when  its  image  lies  not  in  the  fovea,  but  a  few 
degrees  away  from  it.  If  no  increase  of  brightness  is  ob- 
served, it  is  at  least  difficult  to  trace  any  decrease  in  bright- 
ness till  the  image  is  many  degrees  from  the  fovea.  This 
experiment  is  most  easily  made  at  night  with  faint  stars. 
In  the  laboratory  it  may  be  made  with  the  dark  box.  On 
the  rear  wall  of  the  box  place  in  a  horizontal  line  three 
bits  of  white  paper  of  equal  size,  at  such  distances  that  the 
line  of  sight  moves  through  an  angle  of  ten  degrees  in  turn- 
ing from  the  middle  one  to  either  of  the  outer  ones.  Make 
a  pin-hole  above  and  below  the  middle  piece,  distant  from  it 
about  an  inch,  and  cover  the  holes  on  the  outside  with 
paper  till  the  holes  are  barely  visible  after  the  eye  has  been 
some  time  adapted.  These  bright  points  serve  to  steady 
the  eye.  The  eye  should  not,  however,  be  directly  fixed 
upon  them,  but  at  a  point  midway  between  them.  Reduce 
the  illumination  of  the  box  to  a  minimum  (e.  g.,  to  the 
amount  of  light  that  would  enter  through  a  pin-hole  cov- 
ered with  one  or  more  pieces  of  porcelain  or  translucent 
cards),  wrap  the  head  and  the  end  of  the  box  in  an  opaque 
cloth,  and  allow  the  eyes  to  become  adapted  to  the  darkness, 


137]  SENSATIONS    OF  LIGHT  AND   COLOR.  135 

looking  from  time  to  time  for  the  shimmer  of  the  papers  at 
the  back  of  the  box.  Full  adaptation  requires  a  long  time, 
but  fifteen  minutes  is  sufficient  in  this  case.  By  degrees,  if 
the  illumination  is  of  the  right  intensity,  the  papers  will  be 
seen  very  faintly.  If  the  eye  is  turned  directly  towards 
one  of  them,  it  often  disappears  in  the  retinal  light  while 
the  others  brighten.  Fixate  each  of  them  successively,  and 
compare  its  brightness  with  the  others ;  fixate  also  other 
points  in  the  field  so  as  to  bring  the  images  upon  different 
quadrants  of  the  retina.  Close  the  eyes  from  time  to  time 
to  renew  the  adaptation,  and  avoid  observations  when  the 
retinal  light  is  strongly  concentrated  in  the  centre  of  the 
field. 

On  the  results  of  such'  experiments  as  this,  and  on  the  explana- 
tion of  the  phenomenon  observed,  experimenters  are  somewhat  at 
variance,  but  see  Helmholtz,  A,  268;  Aubert,  A,  495,  B,  89  ff.;  A. 
E.  Fick,  B;  Kirschmann,  B;  Treitel,  and  the  literature  cited  by 
them. 

137.  Vision  with  Peripheral  Portions  of  the  Retina: 
Perception  of  Color.  The  distribution  of  the  sensibility  of 
the  retina  for  color  is  unlike  that  for  light.  At  the  very 
centre  the  pigment  of  the  yellow  spot  itself  interferes  some- 
what with  the  correct  perception  of  mixed  colors  (see  Ex. 
115).  In  a  zone  immediately  surrounding  this  all  colors 
can  be  recognized.  Outside  of  this  again  is  a  second  zone 
in  which  blue  and  yellow  alone  can  be  distinguished,  and 
at  the  outermost  parts  not  even  these,  all  colors  appearing 
black,  white,  or  gray.  The  zones  are  not  sharply  bounded, 
but  blend  into  one  another,  their  limits  depending  on  the 
intensity  and  area  of  the  colors  used.  The  fixing  of  the 
boundaries  of  the  zones  of  sensibility  is  known  as  perimetry 
or  cam  pi  met  ry. 

a.  With  the  apparatus  at  hand,  find  at  what  angles  from 
the  centre  of  vision  on  the  vertical  and  horizontal  meridians 


136         LABORATORY  COURSE  IN  PSYCHOLOGY.       [137 

of  the  eye  the  four  principal  colors,  red,  yellow,  green,  and 
blue,  can  be  recognized ;  try  white  also.  Keep  the  eye 
steadily  fixed  on.  the  fixation  mark  of  the  instrument,  and 
have  an  assistant  slide  the  color  (say  a  bit  of  colored  paper 
5  mm.  square  pasted  near  the  end  of  a  strip  of  black  card- 
board an  inch  wide)  slowly  into  the  field  from  the  outside. 
It  will  be  well  to  move  the  paper  slowly  to  and  fro  at  right 
angles  to  the  meridian  on  which  the  test  is  made,  so  as  to 
avoid  retinal  fatigue.  Take  a  record  of  the  point  at  which 
the  color  can  first  be  recognized  with  certainty.  Repeat 
several  times  and  average  the  results.  The  size  of  the 
colored  spot  shown  should  be  constant  for  the  different 
colors,  and  the  background  (preferably  black)  against  which 
the  colors  are  seen  should  remain  the  same  in  all  the 
experiments. 

b.  Repeat  the  tests  with  colored  squares  20  mm.  on  the 
side,  and  notice  the  earlier  recognition  of  their  color  as  they 
approach  from  the  periphery. 

c.  Try  bringing  slowly  into  the  field  (best  from  the  nasal 
side)  bits  of  paper  of  various  colors,  especially  violet,  pur- 
ple, orange,  greenish  yellow,  and  greenish  blue ;  or  better, 
hold  the  bit  of  paper  somewhat  on  the  nasal  side  of  the 
field  and  turn  the  eye  slowly  toward  it,  beginning  at  a  con- 
siderable angle  from  it.     If  the  paper  is  held  before  a  back- 
ground containing  a  line  along  which  the  eye  can  approach 
the  paper,  the  eye  will  be  assisted  in  making  the  approach 
gradual ;    the  apparatus  used  in  Ex.  113  b  can  easily  be 
adapted  for  this  purpose.     Observe  that  on  the  outer  parts 
of  the  retina  these  colors  first  get  their  yellow  or  blue  com- 
ponents, and  only  later  the  red  or  green.     If  the  range  of 
choice  is  sufficiently  large,  it  may  be  possible  to  find  a  red 
(inclined  toward  red-purple)  and  a  green  (inclined  toward 
the  blue),  which,  like  pure  blue  and  yellow,  change  only  in 
saturation  and  not  at  all  in  color-tone  as  they  move  inward 


139]  SENSATIONS   OF  LIGHT  AND  COLOR.  137 

toward  the  centre  of  the  field.     These  four  colors  are  the 
Urfarben  or  primary  colors  of  Hering. 

Helmholtz,  A,  372-374,   Fr.  399-400;    Hess,  A;    Hering,  G,  L  ; 

A.  Fick,  A,  B,  206  ff. ;  A.  E.  Fick,  B,  479  ff. ;  Aubert,  A,  539-546,  B, 
116  if. ;  Kirschmann,  C. 

138.  Changes  in  Color-Tone.     In  the  spectrum,  change 
of  wave-length,  if  not  too  small,  is  accompanied  by  change 
of  color-tone.     The  change  is   most   rapid   in  the  yellow- 
green  and   blue-green  regions  of  the  spectrum,  less  rapid 
toward  the  ends,  and  at  the  extreme  ends  the  only  changes 
are  those  in  brightness.  /  With  the  spectroscope  and  day- 
light find  the  characteristic  Fraunhofer  lines  Z>,  JS,  F,  G,  and 
H.     The  D  line  lies  in  the  golden  yellow,  F  in  the  greenish 
blue,  and  H  at  the  end  of  the  violet.     Between  D  and  F  the 
wave-length  changes  from  589.2  to  486.1  w  (from  5.092  X 
1014  to  6.172  X  1014  vibrations  per  second),  and  the  color  runs 
through  yellow  and  green  to  blue,  while  from  F  to  H  with 
the  nearly  proportional  change  in.  wave-length  from  486.1 
to  393.3  w  (from  6.172  X  101*  to  7.628  X  1014  vibrations 
per  second)  the  change  is  only  from  greenish  blue  to  violet. 
Notice  the  region  from  near  the  line  G  to  the  end  of  the 
spectrum  which  shows  little  change  in  color-tone  and  a  simi- 
lar region  of  uniform  color-tone  at  the  red  end.     Notice 
also  the  tendency  of  the  succession  of  spectral  colors  to  return 
upon  itself,  shown  in  the  resemblance  of  the  violet  and  red. 

Helmholtz,  A,  289,320,  Fr.  319  (237);  Wundt,  3te  Aufl.,  I.,  449  f., 
4te  Aufl.,  I.,  485  f. ;  A.  Fick,  B  ;  Aubert,  A,  530  f.  On  just  observ- 
able changes  in  color-tone,  see  B.  O.  Peirce,  Jr.,  Konig  und  Dieterici, 

B,  Brodhun,  A,  and  the  literature  there  cited. 

139.  Changes  in  Saturation.     These  are  easily  shown  on 
the  color-mixer.     Make  a  succession  of  mixtures  of  red  and 
white,   beginning   with   a   proportion   of    white    that   just 
changes  the  red,  and  increase  the  proportion  till  no  effect 
of  red  remains.     At  first  use  a  small  disk  of  red  laid  on 


138         LABORATORY  COURSE  IN  PSYCHOLOGY.        [140 

over  the  larger  disks  as  a  sample  with  which  to  compare  the 
mixtures.  Toward  the  end  of  the  experiment  exchange  the 
red  for  a  small  white  disk.  Notice  the  changes  of  color- 
tone  that  are  to  be  observed,  especially  when  the  amount  of 
color  is  small.  Try  similarly  with  the  other  chief  colors. 
According  to  Rood,  who  worked  with  the  color-mixer,  yellow- 
green  and  violet  are  unchanged;  Helmholtz's  results  with 
spectral  colors  are  somewhat  different. 

Changes  in  saturation  can  also  be  made  by  adding  gray 
of  any  shade  instead  of  white.  The  whole  range  of  mix- 
tures can  be  shown  on  a  single  disk,  like  that  in  Ex.  141,  by 
painting  the  star  upon  a  white  or  gray  ground,  or  by  past- 
ing a  star  of  colored  paper  on  such  a  ground.  With  white, 
however,  the  rays  of  the  star  must  be  given  a  leaf  shape,  or 
the  color  will  fall  off  too  rapidly  from  the  centre. 

Helmholtz,  A,  322,  470-471,  Fr.  369  (281);  Aubert,  A,  531-532; 
Rood,  A,  39-40,  194-201;  Nichols,  A. 

140.  Changes  in  Intensity :  Black  and  White.  Black  and 
white  are  the  extremes  of  intensity  in  the  series  of  grays. 
The  ordinary  black  and  white  of  conversation  are,  however, 
considerably  short  of  these  extremes. 

a.  Compare  a  bit  of  black  velvet  or  of  black  cardboard 
with  a  still  deeper  black  by  holding  it  in  front  of  the  open- 
ing in  the  dark  box.     Compare,  also,  ordinary  white  paper 
in  diffused  light  with  the  same  in  direct  sunlight,  or  with  a 
brightly  illuminated  white  cloud. 

b.  Just  observable  differences  with  medium  intensities. 
Prepare  a  disk  like  that  shown  in  the  accompanying  cut  by 
drawing  along  a  radius  of  a  white  disk  a  succession  of  short 
black  lines  of  equal  breadth.     Let  the  breadth  of  the  line 
correspond  to  about  one  degree  on  the  edge  of  the  disk. 
Since  the  breadth  of  the  line  is  everywhere  the  same,  it 
will  occupy  a  relatively  greater  angle  as  it  nears  the  centre, 


140] 


SENSATIONS    OF  LIGHT  AND   COLOR. 


139 


When  the  disk  is  set  in  rapid  rotation,  each  short  line 
will  give  a  faint  gray  ring,  those  at  the  outer  edge  being 
very  faint,  those  nearer  the 
centre,  darker.  Find  which 
is  the  faintest  ring  that  can 
be  seen,  and  calculate  the 
proportions  of  black  and 
white  in  it.1  The  ratio  of 
black  to  white  measures 
approximately  the  just  ob- 
servable decrease  in  in- 
tensity below  the  general 
brightness  of  the  disk. 
The  results  of  Helmholtz 
and  Aubert  are  respec- 
tively :  Helmholtz,  1 : 117  to  1 : 167,  Aubert,  1  : 102  to  1 : 186, 
the  differences  depending  on  the  intensity  of  the  general 
illumination  of  the  disk.  Some  wandering  of  the  eyes  is 
helpful,  but  too  rapid  motions  which  tend  to  break  up  the 
even  gray  of  the  rings  must  be  avoided.  It  is  absolutely 
essential  that  the  rotation  be  very  rapid  and  perfectly  free 
from  vibration  —  so  rapid  that  with  moderate  motions  of 
the  eyes  the  uniform  gray  of  the  rings  is  not  disturbed.  If 
great  rapidity  is  impossible,  replace  the  single  black  line 
by  two  of  proportionately  less  breadth  on  opposite  sides  of 
the  disk,  or  by  four  at  90°. 

c.   With  these  very  faint  rings  a  disappearance  and  reap- 
pearance is  to  be  observed  somewhat  like  that  found  for 


1  The  formula  for  the  amount  of  black,  assuming  that  the  radial  line  is  abso. 
lutely  black,  and  taking  some  arbitrary  point,  e.g.,  the  middle,  for  calculation,  is  of 

course — — ,  where  6  is  the  breadth  of  the  radial  line,  and  r  the  distance  of  the 

2nr 

chosen  point  from  the  centre  of  the  disk.  The  black  of  the  lines  is  not  quite  abso- 
lute, even  when  the  blackest  black  paint  is  used.  The  differences  in  sensation 
are  therefore  smaller  than  those  shown  by  the  calculation. 


140         LABORATORY  COURSE  IN  PSYCHOLOGY.        [141 

just  audible  sounds  in  Ex.  61  b.  The  observation  is  most 
conveniently  made,  according  to  Pace,  on  a  disk  of  the  fol- 
lowing dimensions  :  diameter  of  disk,  20  cm.,  width  of  radial 
line,  5  mm.,  length  of  the  short  lines,  5  mm.,  spaces  between 
the  short  lines,  8  mm.,  distance  of  innermost  short  line  from 
the  centre  of  the  disk,  17  mm. 

Helmholtz,  A,  384-393;  Fr.  411-419  (310-316);  Aubert,  A,  487- 
492 ;  on  c,  Pace.  For  references  on  the  just  observable  difference  of 
intensity  with  different  standard  intensities,  see  the  chapter  on 
Weber's  Law  below. 

141.  Changes  in  Intensity  :  Colors.  At  their  maximum 
intensity  all  colors  tend  toward  white  or  yellowish  white. 
Red,  however,  hardly  gets  beyond  the  yellow  ;  green  be- 
comes first  yellow,  then  white,  while  blue  and  violet  easily 
reach  it.  At  their  minimum  intensity  all  colors  appear 
gray  or  black.  ]. 

a.  The  maximum  intensity  may  be  observed  with  spec- 
tral colors,  though  not  entirely  homogeneous  ones,  with  a 
prism  placed  in  the  sunlight  so  that  it  throws  an  extended 
spectrum  on  the  wall.     Hold  a  card,  pierced  with  a  pin-hole, 
before  the  eye,  and  bring  the  eye  successively  into  the  dif- 
ferent colors,  looking  meanwhile  at  the  prism.     Something 
of  the  same  kind  may  be  seen  by  looking  through  pieces  of 
colored  glass  at  the  disk  of  the  sun  behind  a  cloud  (in  which 
case  the  portions  of  the  cloud  seen  at  the  sides  of  the  glass 
afford  a  means  of  comparison),  or  at  the  image  of  the  sun 
reflected  from  an  unsilvered  glass  plate,  or  by  concentrating 
light  from  colored  glass  on  white  paper  with  a  convex  lens. 

b.  The  minimum  intensity  with  spectral  colors  may  be 
observed  with  a  spectroscope.     Adjust  the  instrument   so 
that  the  chief  Fraunhofer  lines  can  be  seen,  and  then  place, 
as  a  source  of  light,  at  a  little  distance  from  the  slit  of  the 
instrument,  a  screen  covered  with  dark  gray  paper  or  black 
velvet.     Though  no  color  remains,  a  little  light  can  be  made 


141]  SENSATIONS    OF  LIGHT  AND  COLOR.  141 

out  —  brightest  in  the  region  before  occupied  by  the  green. 
The  observer  must  envelop  his  head  and  the  ocular  of  the 
instrument  in  an  opaque  cloth,  and  allow  time  for  the  adap- 
tation of  his  eye.  This  colorless  spectrum  probably  repre- 
sents what  is  seen  by  a  totally  color-blind  eye. 

Von  Bezold,  with  whom  this  experiment  originates,  ob- 
served with  gradually  decreasing  intensity  a  falling  out  of 
the  yellows  and  blues  before  the  final  stage  of  .colorlessness 
was  reached.  Konig  doubts  whether  the  red  ever  loses 
its  color  entirely. 

With  pigment  colors  a  convenient  way  is  to  paste  equal 
squares  of  colored  papers  upon  a  piece  of  cardboard,  and 
then  to  place  the  whole  in 
the  dark  box,  and  gradually 
reduce  the  illumination,  or 
starting  with  the  illumina- 
tion at  zero,  gradually  in- 
crease it.  Try  with  both 
black  and  white  cardboard 
as  background.  For  dem- 
onstrational  purposes  a  disk 
like  that  in  the  accompany- 
ing cut  (in  which  the  shaded 
part  stands  for  color,  and 
the  solid  black  for  black)  may  be  used  and  the  whole  series 
of  intensities  shown  at  once.1 

Helmholtz,  A,  402-444  ;  A.  Fick,  B,  200-202  ;  Aubert,  ^4, 532-536  ; 
Kood,  A,  181-194;  C.  S.  Peirce.  On  a,  Helmholtz,  A,  284-285, 
465-466,  Fr.  315  (234);  Brodhun,  B.  On  6,  Helmholtz,  A,  469,  471- 


1  Since  the  black  of  the  disk  is  really  a  very  dark  gray,  and  would  thus  make  a 
change  in  saturation,  this  is  not  an  absolutely  pure  experiment,  but  is  sufficiently 
exact  for  showing  the  general  effect  of  darkening.  If  a  practically  perfect  black 
is  desired,  it  may  be  had,  following  Rood,  by  making  the  colored  star  rotate 
before  an  opening  into  a  dark  room  or  a  suitable  dark  box. 


142         'LABORATORY  COURSE  IN  PSYCHOLOGY.       [143 

472;  von  Bezold,  A-  Ebert;  Abney  and  Festing;  Konig,  A,  354  ff., 
where  other  literature  is  cited. 

For  measurements  of  the  just  observable  difference  of  intensity 
for  different  colors,  see  Helmholtz,  A,  402-415;  Aubert,  A,  531; 
A.  Fick,  -4,  177;  and  the  references  given  by  them. 

142.  Purkinje's   Phenomenon.     In  a  light  of  moderate 
brightness  choose  a  bit  of  red  paper  and  a  bit  of  blue  paper 
that  are  of  about  equal  intensity  and  saturation,  carry  both 
into  full  sunlight  and  notice  which  appears  brightest ;  carry 
both  into  a  darkened  room,  or  place  them  in  the  dark  box 
and  compare  them  again.     If  a  dark  room  or  box  is  not  at 
hand,  observe  them  through  a  fine  pin-hole  in  a  card,  or 
even  with  nearly  closed  eyes. 

Helmholtz,  A,  428-430,  443-444,  Fr.  420-425  (317-321) ;  Hillebrand ; 
Konig,  A-  Charpentier,  A,  227  ff.,  335  ff.;  Rood,  A,  189  ff. 

143.  Size  of  the  Colored  Field.     When  the  retinal  area 
stimulated  is  very  small,  colored  surfaces  appear  colorless, 
with  ordinary  intensities  of  illumination.     When  somewhat 
larger  they  may  appear  colored,  but  not  necessarily  in  their 
true  color-tone.     The  background  against  which  they  are 
placed  is  also  important. 

a.  On  pieces  of  black  and  white  cardboard,  paste  small 
squares  of  several  kinds  of  colored  paper,  one  series  5  mm. 
square,  one  2  mm.  square,  and  one  1  mm.  square.     Walk 
backward  from  them  and  notice  their  loss  of  color.     Ob- 
serve also  the  changes  in  color-tone. 

b.  A  number  of  retinal  impressions,  even  when  not  con- 
tiguous, are  mutually  supportive  in  color  effect.     This  is 
conveniently  shown  in  the  indirect  field.      In  a  two-inch 
square  of  black  cardboard,  punch  sixteen  holes  arranged  in 
the  form  of  a  square,  four  rows  of  four  holes  each.     The 
holes  should  be  an  eighth  or  three-sixteenths  of  an  inch  in 
diameter,  and  be  separated  by  spaces  of  the  same  extent. 
Paste  upon  the  back  of  the  square  a  piece  of  red  paper  of 


144]  SENSATIONS   OF  LIGHT  AND   COLOR.  143 

sufficient  size  to  cover  the  holes,  thus  making  of  them  six- 
teen little  red  circles.  Prepare  also  another  piece  of  black 
cardboard  of  such  shape  that  it  may  be  laid  over  the  square 
and  cover  all  the  holes  except  one  of  the  corner  ones,  and 
again  when  necessary  may  easily  be  removed. 

With  the  apparatus  used  in  Ex.  137,  find  the  point  on  the 
nasal  half  of  the  retinal  horizon  where  the  single  red  circle 
can  just  no  longer  be  seen  in  its  true  color.  In  making  this 
determination,  the  square  should  be  so  held  that  the  diag- 
onal to  which  the  uncovered  circle  belongs  is  horizontal. 
When  the  point  has  been  found,  uncover  the  remaining 
fifteen  circles  (all  farther  toward  the  periphery),  and  notice 
that  the  color  of  the  group  can  be  seen  distinctly.  Fatigue 
in  fixing  the  limit  at  which  the  circle  can  be  seen  should  be 
avoided. 

On  a,  Helniholtz,  A,  374-375,  Fr.  399-400  (300) ;  Aubert,  A,  536- 
539;  Bering,  R,  18.  On  6,  A.  E.  Fick,  A  and  B  (especially  451-452). 

144.  Duration  of  Illumination.  Fechner's  Colors.  The 
retinal  inertia  is  different  for  different  colors.  In  the  ex- 
periments on  after-images  (Ex.  125  d),  it  was  observed  that 
the  after-image  of  a  white  surface  faded  away  through  a 
succession  of  colors ;  a  succession  of  colors  appears  also  to 
result  from  a  very  brief  vision  of  a  white  surface.  This 
can  be  seen  upon  almost  any  slowly  rotating  disk  of  black 
and  white ;  those  used  in  Exs.  128  b  and  145  c  show  the 
colors  well,  and  that  in  Ex.  145  a  shows  something  of  the 
dependence  of  particular  colors  upon  particular  rates  of 
recurrence.  Rotate  any  of  these  disks  with  less  rapidity 
than  that  required  for  a  uniform  gray,  and,  keeping  the 
eyes  steadily  fixed  upon  some  point  of  its  surface,  notice 
both  the  advancing  and  the  retreating  edges  of  the  white 
portions  of  the  disk.  The  colors  may  not  appear  instantly, 
but  are  not  difficult  to  get  with  attentive  gazing. 

Very  striking  and  beautiful  effects  can  be  obtained  by 


144         LABORATORY  COURSE  IN  PSYCHOLOGY.       [145 

substituting  for  the  black  and  white  disk  a  black  one  from 
which  narrow  sectors  have  been  removed.  This  pierced 
disk  is  rotated  before  a  brightly  lighted  background,  e.  g., 
a  sheet  of  white  cardboard  in  full  sunlight,  a  bright  cloud, 
or  the  clear  sky,  and  the  eye  is  brought  very  close  to  the 
disk. 

Helmholtz,  A,  530-533,  Fr.  500-504  (380-383) ;  Fechner,  A ;  Brueke ; 
Exner;  Aubert,J.,560;  Rood,  A,  92  ff.,  B\  Nichols,  B;  Charpentier, 
B  and  C. 

145.  Kate  of  Rotation  Required  for  a  Uniform  Blending 
of  Black  and  White.  All  blending  of  colors  by  rotation 
depends  on  the  phenomenon  of  positive  after-images  (Ex. 
125).  A  disturbance  once  set  up  in  the  retina  does  not  at 
once  subside,  but  continues  an  instant  after  the  removal  of 
the  stimulus.  If  stimuli  follow  in  sufficiently  rapid  succes- 
sion the  disturbances  fuse,  and  the  result  is  the  same  as  if 
the  stimuli  had  been  mixed  before  reaching  the  retina.  A 
rough  determination  of  the  rate  required  for  uniform  blend- 
ing may  be  made  with  the  color-mixer  and  a  metronome. 
a.  Place  the  color-mixer  in  such  a  position  that  the  disk 
(like  that  in  the  margin)  shall 
be  illuminated  by  diffused 
daylight  only.  Turn  the 
driving-wheel  slowly  and 
ascertain,  by  counting,  how 
many  turns  of  the  disk 
correspond  to  one  turn  of 
the  driving-wheel.  Start 
the  metronome,  and  turn 
the  driving-wheel  in  time 
to  its  beats,  making  a  turn 
every  one,  two  or  four  beats. 
Notice  which  of  the  rings,  if  any,  is  just  blended  into  a 
uniform  gray.  If  none  is  just  blended,  change  the  rate  of 


145]  SENSATIONS   Of  LIGHT  AND   COLOtt.  145 

the  metronome  a  little,  and  repeat  the  trial  till  such  a  one 
is  found.  From  the  rate  of  the  metronome,  the  number  of 
turns  of  the  driving-wheel,  and  the  number  of  white  sectors 
in  the  just  blended  ring,  find  the  number  of  stimuli  per 
second  required.  The  experiment  is  easier  when  two 
observers  work  together,  one  giving  his  attention  to  the 
regular  driving  of  the  color-mixer,  and  the  other  to  watch- 
ing the  disk.  The  driving-belt  of  the  instrument  must 
be  tight  enough  not  to  slip,  and  the  metronome  should 
be  kept  well  wound  up.  Its  scale  should  also  be  verified 
by  counting  with  a  watch.  The  observer  must  of  course 
avoid  eye  motions  which  break  up  the  uniformity  of  the 
gray. 

b.  Kepeat  the  determination  with  the  disk  in  direct  sun- 
light ;  also  in  a  partially  darkened  room  or  at  twilight. 

c.  A  disk  like  that  in   the  margin   shows   mixtures  of 
several  different  proportions  of 

black  and  white  at  once.     If 
such  a  disk  is  brought  slowly 
to  the  rate  just  neces- 
sary to  give  a  uniform 
gray  at   the   centre,  a 
little  flickering  can  still 
be  traced  in  the  outer 
rings.     Care  should  be 
taken  not  to  fixate  the 
middle  of  the  disk  ex- 
clusively, for  with  mod- 
erate illumination  the  per- 
iphery of  the  retina  requires  a 
little  greater  speed  for  uniform 

blending  than  the  centre.  Helmholtz  states  that  little  dif- 
ference is  to  be  observed  in  the  rate  at  which  the  flickering 
ceases  with  the  somewhat  similar  disk  shown  at  the  left  in 


146         LABORATORY  COURSE  IN  PSYCHOLOGY.       [147 

Ex.    152  d,  but  with  that  given  here,  it  is  believed  that 
careful  observation  will  not  fail  to  show  a  difference. 

Helmholtz,  A,  488  if.,  Fr.  453  (344)  ff.;  Aubert,  A,  517;  A.  Fick, 
B,  211-222;  Nichols,  B;  Bellarminow  and  the  literature  cited  by 
him. 

146.  The  Talbot-Plateau  Law.     This  law  may  be  stated 
as  follows :  When  once  the  rate  of  rotation  is  sufficient  to 
give  a  uniform  sensation,  the  color  and  brightness  of  any 
given  concentric  ring  of  the  disk  are  the  same  that  they 
would  be  if  all  the  light  reflected  from  it  were  evenly  dis- 
tributed over  its  surface,  and  no  further  increase  in  rapidity 
produces  any  effect  upon  its  appearance.     Rotate  the  disk 
used  in  Ex.  145  a,  and  increase  the  rapidity  till  the  inner- 
most portion  gives  a  uniform  gray.     When  this  appears,  the 
rate  of  recurrence  in  the  outermost  ring  is  32  times  more 
rapid  than  in  the  innermost,  and  yet  no  difference  in  shade 
is  to  be  seen.     To  show  that  the  gray  is  actually  of  the 
same  brightness  that  would  come  from  an  even  distribution 
of  the  light  reflected  from  the  whole  surface  of  the  ring, 
prepare  a  disk  with  many  equal  black  and  white  sectors  — 
32  or  more  of  each.     Place  the  disk  on  the  color-mixer,  and 
look  at  it  when  at  rest  through  a  double  convex  lens  of 
short  focus  (e.g.,  1  in.),  held  at  such  a  distance  from  the  eye 
and  disk  that  no  distinct  image  is  formed,  but  the  field  of 
the  lens  appears  an  even  blur  of  gray.     Now  put  the  disk 
in   rapid   rotation  and    notice  that   the  gray  remains  un- 
changed. 

The  result  of  these  experiments  would  be  the  same  were 
other  colors  substituted  for  black  and  white. 

Helmholtz,  A,  482-485,  Fr.  446-450  (338-341);  Aubert,  A,  515-516; 
Talbot;  Plateau. 

147.  Brticke's  Experiment.     When  the  rate  of  rotation 
is  insufficient  to  produce  an  even  blending,  the  brightness 


148]  SENSATIONS   OF  LIGHT  AND   COLOR.  147 

of  the  disk  is  influenced  by  the  rate.  Set  the  disk  used  in 
Ex.  145  a  in  rapid  enough  rotation  to  blend  the  innermost 
ring,  and  then  let  it  gradually  come  to  rest.  As  it  turns 
more  and  more  slowly,  there  will  be  observed  in  one  ring 
after  another,  beginning  with  the  innermost,  just  as  it  loses 
its  uniform  character,  a  notable  brightening.  The  white 
sectors  now  have  opportunity  to  produce  their  full  effect 
upon  the  retina  before  they  are  succeeded  and  their  impres- 
sion cut  off  by  the  black  sectors. 

Helmholtz,  A,  Fr.  455-456;  Exner;  Aubert,  A,  510. 

COLOR  MIXING. 

148.  Mixed  Colors.  Experiments  upon  this  subject  can- 
not be  regarded  as  entirely  satisfactory  except  when  made 
with  pure  (homogeneous)  spectral  colors.  The  colored 
papers  with  which  the  following  experiments  are  made 
show  anything  but  homogeneous  colors,  as  can  easily  be  seen 
by  looking  at  scraps  of  them  on  a  dark  background  through 
a  prism.  They  produce  the  same  mixture  effects,  however, 
that  spectral  colors  of  the  same  tone,  intensity,  and  satura- 
tion would  produce  ;  and  the  great  facility  of  their  manipu- 
lation on  the  color-mixer  recommends  them  for  preliminary 
experiments  and  for  illustrative  purposes. 

Three  colors  properly  selected  serve  to  produce  by  their 
mixtures  all  the  intermediate  colors  (though  in  most  cases 
in  less  saturation)  with  purple  and  white  (i.  e.,  gray)  in  addi- 
tion. The  colors  generally  selected  are  red,  green,  and  blue 
or  violet.  Green  cannot  be  mixed  from  colors  that  them- 
selves do  not  resemble  it ;  i.e.,  it  can  be  mixed  from  yellow- 
green  and  blue-green,  but  not  from  yellow  and  blue,  and  not 
in  anything  like  full  saturation. 

The  general  facts  of  color  mixing,  together  with  the 
method  of  representing  them  in  a  two  dimensional  diagram, 
were  first  discovered  by  Newton,  and  are  sometimes  desig- 


148         LABORATORY  COURSE  IN  PSYCHOLOGY.       [148 

nated  by  the  general  term  of  Newton's  Law.  For  the 
methods  of  constructing  such  diagrams,  see,  among  others, 
Helmholtz,  A,  334  ff.,  Aubert,  A,  524  ff.,  and  Kood,  J,  218 
ff.,  224  ff. 

a.  Mix  a  yellow  from  red  and  green  on  the  color-mixer. 
The  yellow  produced  will  be  dark,  and,  as  a  test  of   its 
hue,    should   be   matched  with   a   mixture   of  yellow  and 
black  made  with  smaller  disks  set  on  above  the  first.     In 
the  same  way  mix  a  blue  from  green  and  violet  that  shall 
match  a  mixture  of  blue  and  black  (or  blue,  black,  and 
white). 

b.  From  red  and  violet  or  blue,  mix  several  purples  be- 
tween violet  arid  red. 

c.  From  red,  green,  and  violet,  mix  a  gray  that   shall 
match  a  mixture  of  black  and  white  on  the  small  disk.     In 
such   a  case   as   this   it  is  highly  probable  that   the  gray 
appears,  because  the  combined  colors  furnish  among  them 
light  of  all  wave-lengths  in  about  the  proportions  in  which 
they  occur  in  ordinary  white  light.     With  the  homogeneous 
red,  green,  and  violet  of  the   spectrum,  the  case  would  of 
course  be  different.    To  avoid  troublesome  after-images,  the 
adjustment  of  the  disks  should  be  left  to  an  assistant,  or 
the  observer  should  wear  dark  glasses,  except  when  the 
disks  are  in  revolution  at  full  speed. 

If  the  colored  disks  used  in  these  experiments  are  not 
opaque,  several  should  be  used  at  once  instead  of  a  single 
one. 

For  demonstrational  purposes  mixtures  of  two  colors  in 
different  proportions  can  be  shown  on  a  single  disk  of  the 
star  form  (see  Ex.  141)  by  painting  the  star  in  one  color 
and  the  ground  of  the  disk  in  another  (or  by  pasting  colored 
papers  instead  of  painting),  but  in  either  case  some  trial 
will  be  necessary  to  determine  the  proper  shape  for  the 
rays. 


149]  SENSATIONS   OF  LIGHT  AND   COLOR.  149 

Helmholtz,  A,  311-316,  320-322,  325-333,  375,  376-473,  485,  Fr. 
359-365,  367-369,  450  (272-277,  279-281,  341);  Aubert,  A,  521-524- 
Bering,  Jf;  Maxwell,  A  and  B ;  Eood,  A,  124  ff. 

149.  Complementary  Colors.  The  combination  of  red, 
green,  and  violet  mentioned  in  the  last  experiment  is  not 
the  only  combination  that  gives  white  or  gray.  For  every 
color  there  is  another  or  complementary  color,  which,  mixed 
with  it,  gives  a  colorless  combination.  \  Some  of  these  pairs 
are  red  and  blue-green,  yellow  and  indigo-blue,  green  and 
purple,  blue  and  orange,  violet  and  yellow-green. 

a.  Try  several  of  these  pairs  upon  the  color-mixer,  match- 
ing the  resultant  gray  with  a  mixture  of  black  and  white 
on  the  small  disk.  It  will  probably  be  found  in  some  cases 
that  no  possible  proportions  of  the  colored  papers  at  hand 
will  give  a  pure  gray.  In  that  case  a  little  of  the  color 
complementary  to  that  remaining  in  the  gray  must  be 
added.  Suppose  the  red  and  blue-green  papers,  when  com- 
bined, give  gray  with  a  tinge  of  brown  (i.e.,  dark  orange) ;  a 
certain  amount  of  blue  must  then  be  added  to  compensate. 
For  example,  with  certain  papers  180°  of  blue-green  +  36° 
indigo-blue  -4-  144°  red  make  a  gray  that  matches  90°  white 
+  270°  black.  To  see  the  true  complement  of  the  red  used, 
it  is  then  necessary  to  prepare  a  disk  carrying  green  and 
indigo  in  the  proportions  of  180  and  36;  i.e.,  300°  blue- 
green,  60°  indigo.  In  the  same  way  the  complement  of  the 
blue-green  used  is  a  bluer  red  than  that  of  the  red  paper, 
and  may  be  seen  by  itself  by  mixing  288°  red  with  72° 
indigo.  It  is  very  important  here,  and  in  all  cases  where 
a  resultant  white  or  gray  is  to  be  observed,  to  have  some 
undoubted  white  or  gray  in  the  field  to  prevent  mistake  in 
very  faint  tinges  of  color. 

The  criticism  made  upon  Ex.  148  c  applies  here  with 
equal  force.  To  be  conclusive,  the  experiment  must  be 
made  with  far  simpler  colors  than  those  of  colored  papers. 


150         LABORATORY  COURSE  IN  PSYCHOLOGY.       [150 

/b.  Negative  after-images,  when  projected  on  a  white  sur- 
face, are  seen  in  colors  approximately  complementary  to 
those  that  give  rise  to  the  after-images.  Compare  comple- 
mentary colors  found  in  this  way  with  those  found  on  the 
color-mixer. 

Helmholtz,  A,  316-319,  Fr.  365-367  (277-278);  Aubert,  A,  521- 
524;  Konig  und  Dieterici,  A,  284  ff.  ;  Kood,  A,  161  ff. 

150.  Other  Methods  of  Mixing  Colored  Lights,  a.  Lam- 
bert's Method.  The  Reflection  Color-Mixer.  This  is  the 

simplest  of  all  the  methods. 
The  colors  to  be  mixed  are 
placed  on  a  suitable  back- 
ground (e.  g.,  a  smooth  sur- 
face of  black  velvet),  on  op- 
posite sides  of  a  vertical 
glass  plate.  The  eye  is 
brought  into  such  a  position 
that  the  reflected  image  of  the  color  on  one  side  appears  to 
overlie  that  seen  by  transmission  on  the  other  side.  The 
glass  must  of  course  be  of  good  quality  and  clean.  The 
relative  intensity  of  the  colors  can  be  varied  by  varying 
their  distance  from  the  glass.  Bringing  the  colors  near  the 
glass,  or  raising  the  eye,  strengthens  the  reflected  and  weak- 
ens the  transmitted  light.  Strips  of  colored  paper  placed 
with  their  ends  next  the  glass,  provided  the  illumination  is 
equal,  will  show  an  even  blending  of  the  colors  through  a 
considerable  range  of  intensities,  one  color  predominating 
at  one  end  of  the  combined  image,  the  other  at  the  other 
end. 

By  substituting  a  bit  of  glass  on  a  black  background  for 
one  of  the  colors,  and  then  placing  the  instrument  so  that  a 
portion  of  clear  sky  may  be  reflected  in  the  glass,  it  is  possi- 
ble to  mix  sky-blue  with  its  complement,  or  with  any  other 
color. 


150] 


SENSATIONS   OF  LIGHT  AND   COLOR. 


151 


To  mix  two  colors  in  equal  proportions,  arrange  them 
with  black  and  white,  as  in  the  diagram  below.  Adjust 
the  glass  (or  tno  position  of  the  eye)  till  the  grays  made 
by  the  black  and  white  at  the  ends  exactly  match ;  the 
colors  will  then  be  mixed  in  equal  proportions. 


GLASS. 


b.  Mixture  by  Double  Refraction.     Colored  areas  placed 
side  by  side  appear  mixed  when  regarded  through  a  double 
refracting  prism.     The  prism  doubles  both  fields,  and  causes 
a  partial  overlapping.     In  the  overlapped  portion  the  colors 
are  mixed,  each  color  being  present  in  the  mixture  at  ap- 
proximately half  its  original  brightness.     The  prism  should 
be  achromatic. 

c.  Mixture  of  Spectral  Colors.    Fine  mixtures  may  be  ob- 
tained with  a  prism  and  Figs.  1,  2,  and  3  of  Plate  I. ;  or,  still 
better,  from  figures  shaped  like  these,  but  in  white  upon  a 
black  ground.    Since  a  prism  refracts  different  kinds  of  light 
in  different  degrees,  it  produces  a  multitude  of  partially  over- 
lapping images  of  a  bright  object,  which  appear  to  the  eye 
as  colored  fringes.     (Observe  through  a  prism  held  horizon- 
tally, an  inch  square  of  white  paper  on  a  black  background.) 
These  overlapping  images  may  be  illustrated  by  the  follow- 
ing diagram,  in  which  the  horizontal  lines  stand  for  the 


152         LABORATORY   COURSE  IN  PSYCHOLOGY.       [150 

images,  and  the  capital  letters  for  the  colors  of  the  light 
producing  them. 

a          b 


V  -  v 

B  -         —B 

G  -  -  G 


Y 

O 

R 


d  C 

In  the  area  a  b  c  d  all  the  images  overlap  and  the  white 
of  the  paper  is  still  seen.  Toward  the  left  from  a,  however, 
the  different  kinds  of  light  gradually  fail,  beginning  with 
the  red.  The  successive  colors  from  greenish  blue  to  violet 
result  from  the  mixture  of  what  remains.  At  the  other 
end  a  similar  falling  away  of  the  colors  gives  the  succession 
from  greenish  yellow  to  red.  In  Fig.  1,  the  spectra  seen  on 
the  upper  and  lower  edges  of  the  inch  square  of  white 
paper  are  brought  side  by  side  ;  on  one  side  red,  orange, 
and  yellow,  and  on  the  other  greenish  blue,  blue,  and  violet. 
The  colors  that  stand  side  by  side  are  complementary  pairs, 
both  in  tone,  intensity,  and  saturation  ;  for  the  greenish 
blue  is  the  white  of  the  paper  less  the  red,  and  the  blue  the 
same  less  the  red,  orange,  and  yellow,  and  so  with  the  rest  ; 
and  if  the  two  spectra  be  exactly  superposed,  as  can  be 
done  with  an  adaptation  of  the  method  of  b  above,  they 
will  make  precisely  the  white  from  which  they  originated. 

If  a  very  narrow  strip  of  white  upon  a  black  ground  is 
looked  at  through  the  prism,  the  images  overlap  less  and 
another  color  appears  ;  namely,  green,  as  may  be  seen  in  Fig. 
2  on  the  narrow  white  band  between  the  black  bars.  When, 
on  the  other  hand,  a  narrow  black  band  on  a  white  ground 
is  taken,  the  spectrum  of  the  white  surface  above  and  of 
that  below  partially  overlap,  and  give  another  set  of  mix- 
tures. If  the  diagram  is  held  near  the  prism  at  first,  and 


150]  SENSATIONS   OF  LIGHT  AND   COLOR.        .     153 

then  gradually  withdrawn  from  it,  the  advance  and  mixing 
of  the  spectra  can  easily  be  followed.  Besides  the  greenish 
yellow  at  one  end  and  the  greenish  blue  at  the  other,  there 
are  a  rich  purple,  complementary  to  the  green  beside  it,  and 
a  white  between  the  purple  and  the  greenish  yellow.  The 
last  is  a  white  produced  by  the  mixture  of  the  blue  of  one 
spectrum  with  the  complementary  orange-yellow  of  the 
other. 

Fig.  3  shows  a  number  of  color  mixtures  with  different 
proportions  of  the  constituents.  In  the  spectra  from  the 
white  triangle  appear  mixtures  of  each  color  in  the  spectrum 
seen  on  the  white  band  in  Fig.  2,  with  every  other  color 
found  there.  Upon  the  black  triangle  the  spectra  from  the 
white  edges  above  and  below  show  mixtures  similar  to 
those  on  the  black  band  in  Fig.  2.  The  diagram  should  be 
placed  at  such  a  distance  that  a  little  of  the  white  and  black 
triangles  can  still  be  seen. 

Helmholtz,  A,  350-357,  485,  491-493,  Fr.  402-407,  450,  458-461 
(303-306,  341,  347-349);  Aubert,  ^1,521-524  ;  Maxwell,  A-  Rood,  A, 
108  ff.,  124  ff.;  Hering,  O;  von  Bezold,  B,  77  ff.  On  a  and  c,  Ben- 
son. On  refined  methods  of  mixing  spectral  colors,  see  especially 
the  first  reference  to  Helmholtz. 

CONTRAST. 

The  effect  of  one  color  on  another,  when  not  mixed  with 
it,  but  presented  to  the  eye  successively,  or  simultaneously 
in  adjacent  fields,  is  known  as  contrast.  Two  kinds  are 
distinguished,  Successive  contrast  and  Simultaneous  contrast. 
The  color  that  is  changed  or  caused  to  appear  upon  a  color- 
less surface,  is  known  as  the  induced  color ;  the  color  that 
causes  the  change  is  called  the  inducing  color.  Successive  con- 
trast is  largely  a  matter  of  negative  after-images,  and  their 
projection  upon  different  backgrounds,  and  is  universally 
regarded  as  a  matter  of  physiology.  Simultaneous  contrast, 
on  the  contrary,  has  been  regarded  by  Helmholtz  and  his 


154         LABORATORY  COURSE  IN  PSYCHOLOGY.       [151 

supporters  as  a  matter  of  psychology,  as  a  sort  of  mis- 
judgment.  The  studies  of  the  last  few  years,  however, 
chiefly  those  of  Hering,  have  demonstrated  that  simultane- 
ous contrast  also  in  most,  and  probably  in  all  cases,  is 
physiological,  a  phenomenon  of  the  retina  (and  its  central 
connections),  not  of  mistaken  inference. 

151.  Successive  Contrast,  a.  Prepare  a  set  of  colored 
fields  of  the  principal  colors,  including  white,  black,  and 
gray,  say  3x5  inches  in  size,  and  some  small  bits  of  the 
same  colors,  say  1  cm.  square.  Lay  a  small  square  on  the 
black  field,  get  a  strong  negative  after-image,  and  project 
it  first  on  the  white  and  then  on  the  other  fields.  Notice 
that  the  color  of  the  after-image  spot  is  that  of  the  field  on 
which  it  is  projected,  minus  the  color  that  produced  the 
spot ;  e.  g.,  the  after-image  of  red  projected  on  violet  looks 
blue,  and  on  orange  looks  yellow.  Or,  to  say  the  same 
thing  in  other  words,  the  color  of  the  spot  is  a  mixture  of 
the  color  of  the  after-image  with  the  color  of  the  ground 
upon  which  it  is  projected.  \  Thus  a  blue-green  after-image 
when  projected  on  violet,  gives  blue;  when  projected  on 
orange,  gives  yellow.  Notice  that  when  the  image  is  pro- 
jected on  a  field  of  the  inducing  color  it  causes  the  spot  on 
which  it  rests  to  look  dull  and  faded ;  but  when  it  is  pro- 
jected upon  a  field  of  complementary  color,  it  makes  the 
spot  richer  and  more  saturated,  [j  Indeed,  it  is  only  by  first 
fatiguing  the  eye  for  one  color  and  then  looking  at  its  com- 
plement that  the  most  saturated  color  sensations  can  be 
produced.  In  general,  colors  that  are  complementary,  or 
nearly  so,  are  helped  in  appearance  by  contrast ;  those  that 
resemble  each  other  more  nearly  are  injured. 

b.  These  effects,  in  even  greater  brilliancy,  can  be  seen 
by  laying  the  small  square  of  color  directly  on  the  larger 
colored  surface,  staring  at  it  a  few  seconds,  and  then  sud- 
denly puffing  it  away  with  the  breath.  See  also  Ex.  134. 


152]  SENSATIONS   OF  LIGHT  AND   COLOR.  155 

c.  This  contrast  effect  may  be  so  strong  as  actually  to 
overcome  a  moderately  strong  objective  color.  Place  a 
small  piece  of  opaque  orange  paper  in  the  middle^  of  a  pane 
of  red  glass  and  look  through  the  glass  at  a  clear  sky  or 
bright  cloud.  The  strength  of  the  induced  blue-green  will 
be  sufficient  to  make  the  orange  seem  blue.  See  also  Ex. 
124  A 

Helmholtz,  A,  537-542,  Fr.  510-515  (388-392);  Hess,  C  ;  Hood,  A. 
235  ff. 

152.  Mixed  Contrasts.  When  special  precautions  are 
not  taken  to  exclude  successive  contrast,  both  successive 
and  simultaneous  co-operate  in  the  general  effect.  Some  of 
the  results  are  striking  and  beautiful. 

a.  Colored  Shadows.  Arrange  two  lights  so  that  they 
shall  cast  a  double  shadow  of  a  pencil  or  small  rod  upon  a 
white  surface.  The  daylight  will  answer  for  one  light  if  it 
is  not  too  strong,  but  it  must  not  be  forgotten  that  unless 
the  light  conies  from  an  overcast  sky  it  will  be  blue.  In- 
troduce different  colored  glasses  one  after  another  before 
one  of  the  lights,  and  notice  the  beautiful  complementary 
color  that  immediately  appears  in  the  shadow  belonging  to 
that  light.  The  brightness  of  the  two  lights  should  be  so 
regulated  that  the  shadows  shall  be  about  equally  dark 
when  the  colored  glass  is  introduced  before  one  of  the 
lights.  See  also  Ex.  155. 

Use  a  blue  glass,  and  adjust  the  relative  intensities  of  the 
lights  so  that  the  yellow  shadow  appears  at  its  brightest, 
and  notice  that  it  seems  as  bright  as  the  surrounding  blue, 
or  even  brighter.  As  a  matter  of  fact,  however,  it  receives 
less  light  than  the  surrounding  portions  ;  for  in  order  to  be 
a  shadow,  it  must  be  a  portion  of  the  field  from  which  the 
light  is  partly  cut  off. 

#.*  Mirror  Contrasts.  Ragona  Scina's  Experiment.  Place 
upon  the  horizontal  and  vertical  surfaces  of  the  instrument 


156         LABORATORY  COURSE  IN  PSYCHOLOGY.     [152 

white  cards  carrying  black  diagrams.1  The  diagrams  being 
in  place,  hold  between  the  two  at  an  angle  of  45°  a  pane  of 
colored  glass,  say  green,  and  observe  that  the  black  of  the 
horizontal  diagram  seems  tinged  with  the  complementary 
color,  that  is,  purple.  This  contrast  color  may  often  be  im- 
proved by  slightly  altering  the  inclination  of  the  glass,  or 
by  changing  the  relative  illumination  of  the  diagrams  by 
interposing  a  colorless  screen  between  one  or  the  other  of 
them  and  the  source  of  light,  or  by  shifting  the  whole  in- 
strument. This  experiment  will  be  readily  understood  after 

a  consideration  of  the  accompany- 
ing cut.  The  glass  plate  is  repre- 
sented by  C  D,  the  black  portion 
of  the  vertical  diagram  by  the 
projection  opposite  A,  that  of  the 
horizontal  diagram  by  the  projec- 
tion at  B.  The  light  reaching  the 
eye  from  the  white  portion  of  the 
horizontal  diagram  is  colored  green 
by  the  glass ;  that  from  the  white 
portion  of  the  vertical  diagram  is  reflected  from  the  upper 
surface  of  the  plate,  and  is  therefore  uncolored.2  The  mix- 
ture of  the  two  gives  a  light  green  field.  For  simplicity, 
we  may  assume  that  no  light  comes  from  the  black  portions 
of  the  diagram.  Then  in  the  portion  of  the  light  green 

1  Any  black  spot  will  answer.  For  this  experiment  diagrams  made  up  of  sets 
of  heavy  concentric  black  rings,  lines  a  quarter  of  an  inch  wide,  separated  by 
white  rings  of  triple  width,  give  an  excellent  effect.  The  diameters  should  be  so 
chosen  that  a  black  ring  on  the  horizontal  diagram  shall  correspond  to  a  Avhite 
one  on  the  vertical  and  vice  versa,  and  shall  appear  to  lie  in  the  midst  of  the 
white  when  the  diagrams  are  combined  in  the  way  described  above.  A  pair  of 
diagrams  made  up  of  parallel  black  bars,  a  quarter  of  an  inch  wide,  separated 
by  quarter  inch  spaces,  and  so  placed  in  the  instrument  that  they  give  a  checker- 
board pattern  when  combined,  are  useful  for  keeping  in  the  field  a  true  black 
with  which  the  changed  colors  can  be  compared.  * 

1  As  a  matter  of  fact,  a  small  portion  is  also  reflected  from  the  lower  surface 
of  the  glass,  and  contributes  a  minute  amount  of  green. 


152]  SENSATIONS  OF  LIGHT  AND    COLOR.  157 

field  corresponding  to  the  black  of  tlie  vertical  diagram,  the 
white  component  will  be  wanting  and  the  green  will  appear 
undiluted  ;  in  the  portion  corresponding  to  the  black  of  the 
horizontal  diagram,  the  green  component  will  be  wanting 
and  the  faint  white  (i.  e.,  gray)  should  appear  by  itself. 
It  does  not,  however,  because  of  the  contrast  color  induced 
upon  it.  As  a  matter  of  fact,  the  black  portions  are  not 
absolutely  black  ;  the  small  amount  of  light  that  comes 
from  them  tends  on  one  hand  to  make  the  green  image  (im- 
age of  the  black  of  the  vertical  diagram)  a  little  whiter,  and 
on  the  other  hand  to  counteract  the  contrast  in  the  purple 
image  by  adding  to  it  a  little  green.  Try  the  experiment 
with  other  glasses  than  green. 

Another  form  of  the  mirror  contrast  experiment  is  as 
follows.  Place  a  mirror  where  the  sky  or  a  white  surface 
of  some  kind  will  be  seen  reflected  in  it.  Lay  upon  its  sur- 
face a  plate  of  colored  glass  (green  for  example),  and  hold 
a  little  way  above  it  a  narrow  strip  of  black  cardboard  or  a 
pencil.  Two  images  will  be  seen  :  one  a  vivid  green,  the 
other  a  complementary  purple.  The  green  image  belongs 
to  the  surface  reflection  of  the  colored  glass,  as  may  be 
proved  by  observing  that  when  the  strip  of  cardboard 
touches  the  surface,  the  green  image  touches  it  also.  The 
purple  image  belongs  to  the  reflection  from  the  back  of 
the  mirror.  It  is  easy,  by  substituting  a  gray  strip  for  the 
black,  to  show  that  contrast  can  suppress  a  weaker  objective 
color  actually  present.1 

c.  Meyer's  Experiment.  Lay  on  a  large  colored  field  a 
small  piece  of  gray  or  even  black  paper  (e.g.,  1  cm.  wide  by 
2  cm.  long),  and  cover  the  whole  with  a  piece  of  semi- 
transparent  white  paper  of  the  same  size  as  the  colored 
field.  The  contrast  color  will  appear  on  the  gray  paper. 

1  For  fuller  explanation  with  diagram,  see  American  Journal  of  Psychology, 
V.,  1892-93,  407,  and  von  Bezold,  154  f. 


158       LABORATORY  COURSE  IN  PSYCHOLOGY.      [152 

If  thin  tissue  paper  is  used,  more  than  one  thickness  may 
be  needed  for  the  best  result.  Paper  mats,  woven  one  way 
of  gray  paper  and  the  other  of  colored,  show  this  contrast 
beautifully.  They  may  easily  be  made  from  kindergarten 
materials. 

d.    Mixed  Contrasts  with  the  Color-mixer.    Disks  made  on 
the  pattern  of  the  cut  at  the  left  show  beautiful  contrasting 


grays.  The  disk  used  in  Ex.  145  c  shows  a  longer  series, 
but  requires  a  more  rapid  rate  of  rotation.  The  same  can 
be  shown  also  by  laying  a  number  of  small  sheets  of  tissue 
paper  over  one  another  in  such  a  way  that  they  partially 
overlap,  making  a  portion  where  there  is  but  a  single  thick- 
ness, and  next  it  a  portion  where  there  are  two  thicknesses, 
and  next  that  again  one  of  three  thicknesses,  and  so  on. 
When  the  whole  is  held  up  to  the  light,  the  contrasts  of 
adjacent  portions  are  very  easily  seen. 

Contrast  colors  can  be  shown  finely  with  disks  like  that 
in  the  cut  at  the  right,  in  which  the  shaded  portions  repre- 
sent color,  the  black  portions,  black,  and  the  white,  white. 
A  little  care  is  necessary  in  fixing  the  proportions  of  the 
color  to  white  and  black  in  the  disks,  but  in  general  the 


153]  SENSATIONS   OF  LIGHT  AND   COLOR.  159 

brightness  of  the  gray  should  be  about  that  of  the  color. 
When  the  contrast  color  has  been  satisfactorily  obtained, 
bring  near  it  a  piece  of  white  cardboard  (e.g.,  3x5  in.),  so 
held  with  reference  to  the  source  of  light  that  it  appears 
about  as  bright  as  the  contrast  ring.  Hold  the  card  so  that 
its  shadow  does  not  fall  on  the  disk,  or  at  least  is  out  of 
sight.  Notice  the  retreat  of  the  contrast  color  from  its 
edges.  On  such  experiments  as  this  much  stress  is  laid  by 
Helmholtz  and  the  supporters  of  the  psychological  explana- 
tion of  contrast. 

Contrasts  with  two  colors  at  once  can  be  shown  by  mak- 
ing the  inner  portion  of  the  colored  sectors  of  one  color,  the 
outer  portion  of  another.  A  temporary  disk  for  showing 
contrast  effects  may  be  arranged  by  putting  on  the  spindle 
of  the  color-mixer  first  a  large  colored  disk  (e.g.,  20  cm. 
in  diameter),  then  smaller  combined  disks  of  black  and 
white  (e.g.,  12  cm.  in  diameter),  and  finally  a  still  smaller 
colored  disk  (e.g.,  10  cm.  in  diameter). 

Helmholtz,  A,  542  ff.,  Fr.  515-546  (392-417);  Hering,  E\  Aubert, 
496  ff.,  546  ff.;  von  Bezold,  144-171;  Rood,  241-272;  Mayer. 

For  particular  experiments,  see  the  following:  on  a  (second  part), 
von  Bezold,  J5,  153-154;  on  b  (second  part),  Dove;  on  c,  Meyer. 

For  quantitative  measurements  of  contrast  in  grays,  see  Ebbing- 
haus,  B  ;  Lehmann;  and  Kirschmann,  D. 

153.    Some  of  the  Conditions  that  Influence  Contrast. 

a.  Contrasts  are  stronger  when  the  colors  are  near  to- 
gether. Lay  a  bit  of  white  paper  on  a  black  surface,  e.g., 
a  piece  of  black  velvet,  and  notice  that  the  paper  is  whiter 
and  the  velvet  blacker  near  the  margin  of  the  paper  than 
elsewhere,  notwithstanding  that  the  eye  moves  about  freely. 
This  has  received  the  name  of  "  Marginal  contrast "  (Rand- 
contrast). 

On  a  piece  of  gray  paper,  the  size  of  a  letter-sheet,  lay  two 
strips  of  colored  paper  close  side  by  side  (e.g.,  pieces  of 


160        LABORATORY   COURSE  IN  PSYCHOLOGY.       [153 

red  and  yellow  or  of  green  and  blue,  1  cm.  wide  by  4  cm. 
long).  Below  them  to  the  right  and  left,  as  far  apart  as  the 
paper  will  permit,  lay  two  other  strips  of  the  same  size  and 
color,  red  on  the  red  side  of  the  former  pair,  yellow  on  the 
yellow  side.  Notice  the  effect  of  the  difference  in  distance 
on  the  contrasting  pairs.  Contrast  of  this  sort  is  at  a  maxi- 
mum when  one  color  entirely  surrounds  the  other. 

b.  Effect  of  size.     When  the  area  of  the  inducing  color  is 
large  and  that  of  the  induced  color  is  small,  the  contrast 
is  shown  chiefly  on  the  latter ;   when  the  two  areas  are  of 
about  equal  size,  as  in  a  above,  the  effect  is  mutual.     Try 
with  large  and  small  bits  of  paper  upon  a  colored  field. 

c.  Borders    and  lines    of  demarcation  that  separate  the 
contrasting  areas  tend  to  lessen  the  effect  by  excluding  mar- 
ginal contrast ;   and  (since   the  eye  tends  to  move  along 
rather  than  across  strongly  marked  lines),  by  hindering  such 
motions  of  the  eye  as  would  bring  about  successive  contrast. 
Kepeat  Ex.  152  c,  using  two  slips  of  gray  paper  5  mm.  wide 
by  2  cm.  long,  and  substituting  a  piece  of  moderately  trans- 
parent letter-paper  for  the  tissue  paper.    When  the  contrast 
color  has  been  observed,  trace  the  outline  of   one  of  the 
slips  with  a  fine  ink  line  upon  the  paper  that  covers  it,  and 
notice  that  the  color  nearly  or  quite  vanishes.     A  disk  like 
that  in  the  cut  accompanying  Ex.  152  d,  when  provided  with 
a  second  contrast  ring,  marked  off  on  both  its  edges  with  a 
firm  black  line,  shows  a  weakening  of  the  induced  color  in 
the  bordered  ring. 

This  experiment  and  others  like  it  play  an  important  part 
in  the  psychological,  as  opposed  to  the  physiological,  expla- 
nation of  simultaneous  contrast ;  see  Helmholtz,  A,  543  ff., 
559  f.,  Fr.  533  f.,  539,  542,  (406  f.,  411,  414).  Such  a  black 
border  will,  however,  also  make  a  weak  objective  color 
invisible. 

d.  Saturation.     Contrast  effects  are  generally  most  strik- 


154]  SENSATIONS  OF  LIGHT  AND  COLOR.  161 

ing  with  little  saturated  colors.  Compare  the  effect  of 
increasing,  decreasing,  and  extinguishing  the  second  non- 
colored  light  in  the  colored  shadow  experiments.  It  is 
necessary,  however,  to  see  to  it  that  reflected  light  from  the 
walls  and  surrounding  objects  does  not  complicate  the  ex- 
periment. 

Compare  the  intensity  of  the  contrasts  in  Meyer's  experi- 
ment (Ex.  152  c)  before  and  after  the  application  of  the 
tissue  paper.  Notice  also  the  part  played  by  the  white 
light  mixed  with  the  colored  light  in  the  mirror  contrast 
experiments  above.  Try  the  effect  of  introducing  white  or 
black  or  both  into  the  largest  and  smallest  disks  in  the 
arrangement  mentioned  at  the  end  of  Ex.  152.  Powerful 
contrasts  with  the  most  saturated  colors  can  be  observed, 
however,  when  the  proper  conditions  are  fulfilled. 

e.  Colors  induced  upon  gray  fields  are  stronger  when  the 
gray  has  about  the  same  brightness  as  the  inducing  color. 
Repeat  Meyer's  experiment,  using  white  paper  instead  of 
the  gray  or  black.  With  the  three  disk  arrangement  try 
the  effect  of  making  the  intermediate  disk  all  white  and  all 
black.  Eood  finds  that  grays  slightly  darker  than  the 
inducing  color  are  advantageous  when  the  inducing  color 
is  red,  orange,  or  yellow,  and  slightly  lighter  when  the 
inducing  color  is  green,  blue,  violet,  or  purple.  • 

On  conditions  in  general,  see  Helmholtz,  A,  540-541,  Fr.  513-514, 
(390-391),  Kirschmann,  D.  In  Hering,  E,  will  also  be  found  much  on 
the  effect  of  various  conditions.  On  &,  Exner,  B.  On  c,  Helmholtz, 
A,  546-547,  Fr.  539-542  (411-414).  On  d,  Helmholtz,  A,  Fr.  523-524 
(399-400).  On  e,  Rood,  A,  261. 

154.  The  Halo  or  Lichthof  of  Hering.  Contrast  is  often 
to  be  seen  in  negative  after-images.  That  observed  in  after- 
images of  white  objects  on  a  dark  ground  has  been  adduced 
by  Hering  as  an  argument  against  the  psychological  expla- 
nation of  contrast.  Some  of  the  simpler  experiments  are 


162       LABORATORY  COURSE  IN  PSYCHOLOGY.      [155 

as  follows ;  for  his  development  of  them  consult  Hering,  A. 

a.  Lay  a  half  inch  square  of  white  paper  on  a  large  sheet 
of  black  cardboard  (or  better  of  black  velvet),  and  put  a 
small  dot  at  its  centre.     Stare  with  unmoved  eyes  at  the 
dot  for  from  15  to  30  seconds  or  more,  then  close  and  cover 
the  eyes.     There  will  then  be  seen,  neglecting  incidental 
color  effects,  the  dark  after-image  of  the  paper  surrounded 
by  a  halo  of  light,  brightest  next  the  paper  and  gradually 
falling  off  in  brilliancy  toward  the  periphery.     This  is  ex- 
plained on  the  psychological  theory  as  due  to  contrast  with 
the  deep  black  of  the  after-image  of  the  square.     When, 
however,  the  converse  of  the  experiment  is  properly  made 
(a  black  square  on  a  white  ground),  the  dark  halo  which 
would  be  expected  by  contrast  is  not  found,  though  the 
after-image  of  the  black  square  is  very  bright. 

b.  Lay  two  white  squares  side  by  side  two  or  three  milli- 
meters apart   on   the   dark   ground   and   between   them  a 
minute  clipping  of  paper  for  a  fixation  point.     Secure  the 
after-images  as   before.      The   halos   of    the   two    squares 
coincide  in  the  narrow  space  between  and  give  a   much 
brighter  band  in  the  after-image.     Under  favorable  circum- 
stances this  bright  band  may  remain  visible  while  the  after- 
images of  the  squares  themselves  are  temporarily  invisible. 
In  both  these  experiments  it  is  better  to  use  both  eyes  than  a 
single  one.     The  explanation  of  the  halo  as  a  matter  of  false 
judgment,  especially  in  the  last  mentioned  case,  is  not  easy. 

Hering,  A. 

155.  Simultaneous  Contrast  with  Colored  Shadows.  The 
effects  of  simultaneous  contrast  are  almost  always  lost  in  the 
more  powerful  ones  of  successive  contrast.  The  first  requi- 
site, therefore,  of  an  experiment  on  the  first,  is  the  exclusion 
of  the  second.  This  is  not  difficult  for  colored  shadows. 

a.  Place  a  good-sized  piece  of  white  paper  on  a  table  in 
such  a  position  that  it  may  be  illuminated  at  the  same 


155]  SENSATIONS   OF  LIGHT  AND   COLOR.  163 

time  from  a  window  (if  the  day  is  overcast)  and  from  a  gas- 
jet.  Set  upon  it  a  small  block  or  other  object  (about  5  cm. 
by  10  cm.  in  size)  ;  something  black  in  color  is  best.  Light 
the  gas  and  observe  the  two  shadows,  one  cast  by  the  light 
from  the  window,  the  other  by  the  gas.  The  first  will 
appear  yellowish,  the  second  clearly  blue.1  Adjust  the  dis- 
tance and  position  of  the  block  with  reference  to  the  light 
so  that  the  shadows  shall  appear  about  equally  dark,  and 
the  blue  shadow  shall  be  as  sharply  bounded  as  possible, 
and  for  that  purpose  it  is  well  to  have  the  shadow  cast  by 
the  edge  rather  than  the  flat  side  of  the  flame.  The  color 
of  the  yellowish  shadow  is  objective  and  due  to  the  yellow 
of  the  gas-flame,  that  of  the  blue  is  due  to  the  contrast,  but 
largely,  as  yet,  to  successive  contrast.  Put  a  dot  in  the 
centre  of  the  blue  shadow,  to  serve  as  a  fixation-point,  and 
another  on  the  edge.  Fasten  a  paper  tube  (preferably 
blackened  inside)  so  that  it  can  easily  be  shifted  from  one 
dot  to  the  other.  Cut  off  the  gas-light  by  holding  a  card 
between  it  and  the  block  ;  adjust  the  tube  so  that  the  dot  in 
the  middle  of  the  shadow  may  be  fixated  without  any  of 
the  field  outside  of  the  shadow  being  seen.  Wait  until  all 
of  the  blue  has  disappeared  from  the  shadow,  and  then, 
still  looking  through  the  tube,  remove  the  card.  The  field 
remains  entirely  unchanged  and  appears,  as  before,  a  color- 
less gray.  The  former  blue  color  is  thus  shown  to  be  sub- 
jective and  due  to  contrast  with  the  yellow  lighted  area  in 
which  it  lies. 


1  This  setting  of  the  experiment  succeeds  best  when  the  daylight  is  weak,  as, 
for  example,  just  before  the  lights  are  usually  lighted  in  the  evening.  If  the  ex- 
periment is  to  be  made  in  broad  day,  the  light  must  be  reduced  by  curtains  or 
otherwise;  if  at  night,  there  must  be  two  lights,  one  corresponding  to  the  win- 
dow and  one  to  the  gas,  and  the  latter  must  shine  through  a  pane  of  colored  glass. 
If  yellow  glass  is  used,  the  colors  will  be  the  same  as  those  in  this  experiment,  the 
free  flame  taking  the  place  of  the  daylight.  If  the  sky  is  clear,  its  light  is  itself 
blue,  and  would  complicate  the  experiment  somewhat,  Its  light  may,  however, 
be  passed  through  colored  glass  or  gelatine,  but  then  the  orange  color  of  the 
gas-light  must  be  regarded. 


164       LABORATORY  COURSE  IN  PSYCHOLOGY.      [l56 

b.  Cut  off  the  gas-light  again  and  adjust  the  tube  so  that 
the  dot  in  the  edge  of  the  shadow  may  be  fixated.  Taking 
great  care  not  to  move  the  eye,  withdraw  the  card.  The 
part  of  the  field  of  the  tube  filled  by  the  shadow  will  ap- 
pear bluish,  that  of  the  remainder  reddish  yellow.  After 
a  little  time  of  steady  fixation,  cut  off  the  gas-light  once 
more  and  observe  the  instant  reversal  of  the  colors.  The 
shadow  now  appears  in  reddish  yellow,  the  rest  of  the  field 
blue.  The  color  of  the  shadow,  both  before  and  after  the 
final  interposition  of  the  card,  is  due  to  simultaneous  con- 
trast, in  the  first  case  with  the  reddish  yellow  light,  and  in 
the  second  with  its  after-image. 

Helmholtz  and  his  supporters  explain  all  cases  of  simul- 
taneous contrast  as  errors  of  judgment ;  in  the  case  of  the 
colored  shadow,  for  example,  we  mistake  the  yellow  of  the 
gas-lighted  field  for  white,  and  consequently  find  the  shadow 
which  is  really  gray  to  be  bluish.  In  the  case  of  this  par- 
ticular experiment,  Hering  and  Delabarre  have  shown  this 
psychological  explanation  unnecessary  and  a  physiological 
one  all  sufficient,  and  Hering  has  done  the  same  for  other 
forms  of  experiments. 

On  simultaneous  contrast  in  general,  see  Helmholtz,  A,  542  ff., 
Fr.  515-547  (392-418);  Hering,  A  and  E.  On  colored  shadows  see 
Helmholtz,  A,  551-553,  Fr.  517-519  (394-396);  Hering,  E  ;  Delabarre. 

On  Helmholtz' s  theory  see  Helmholtz,  J.,  543  ff.,  Fr.  516,  533-538 
(392,  407-411);  Hering,  E ;  Rood,  A,  252  ff.;  von  Bezold,  B,  146  ff. 

For  quantitative  measurements  of  simultaneous  contrast  under 
various  conditions,  see  Kirschmann,  D. 

156.    Simultaneous  Contrast.   Hering's  Binocular  Method. 

a.  Set  a  red  glass  in  the  right  frame  of  the  binocular 
color-mixer,  a  blue  glass  in  the  left.  Look  fixedly  through 
the  colored  glasses  at  the  cork  ball  below,  bringing  the  eyes 
close  to  the  glasses  and  the  nose  between  them.  Adjust  the 
side  screens  till  the  white  ground  below  appears  in  a  uni- 


157]  SENSATIONS   OF  LIGHT  AND   COLOR.  165 

form  light  violet  from  the  binocular  mixture  of  the  red  and 
blue  (see  Ex.  167).  The  narrow  strip  of  black  paper  011  the 
white  is  seen  double,  the  right  hand  image  bluish,  the  left 
yellowish. 

b.  The  possibility  of  successive  contrast,  however,  is  not 
yet  excluded.  Lay  a  sheet  of  black  paper  over  the  whole 
of  the  white  field  and  its  black  strip ;  rest  the  eyes ;  and 
finally,  when  everything  is  in  readiness,  and  the  eyes  again 
fixed  on  the  ball,  swiftly  draw  away  the  black  paper,  keep- 
ing the  eyes  motionless.  The  contrast  colors  are  seen  on 
the  instant,  before  any  motions  of  the  eyes  that  might  intro- 
duce successive  contrast  have  been  made. 

Hering  argues  that  this  experiment  is  conclusive  against 
the  psychological  explanation  of  simultaneous  contrast, 
unless  a  separate  unconscious  judgment  is  to  be  made  for 
each  eye ;  for  that  which  is  seen  is  a  light  violet  field, 
and  the  contrast  color  to  that  should  be  a  greenish  yellow, 
and  both  images  of  the  strip  should  be  alike,  whereas, 
actually,  the  images  appear  in  different  colors,  neither  of 
which  is  the  color  required. 

Hering,  J. 

157.  Induction  of  a  Like  Color.  An  effect  the  reverse 
of  the  ordinary  contrast  effects  sometimes  appears,  the  in- 
ducing color  reappearing  in  the  induced  field. 

a.  Place  close  side  by  side  a  large  piece  of  black  paper 
and  an  equal  sized  piece  of  white.     Make  a  dot  as  a  fixation 
point  at  the  middle  of  their  line  of   junction,  and   stare 
fixedly  at  it  for  half  a  minute.     After  a  few  seconds  the 
w^hite  will  appear  decidedly  darker  and  the  black  decidedly 
lighter,   the   effect   becoming   more  marked   as  fixation  is 
continued.     See  also  Ex.  122. 

b.  A  darkening  or  brightening  of  a  colored  ground  is  often 
to  be  observed  when  a  figure  in  black  or  white  is  placed 


166        LABORATORY  COURSE  IN  PSYCHOLOGY.      [158 

upon  it.  This  is  a  method  of  obtaining  shades  and  tints 
often  used  in  polychromatic  decoration.  Observe  the  effect 
in  Fig.  4  of  Plate  I.  The  same  may  be  observed  occasion- 
ally in  plaid  fabrics,  and  is  shown  very  satisfactorily  in 
kindergarten  mats  woven  in  checker-board  pattern  of  col- 
ored and  gray  papers.  If  a  set  of  graded  grays  is  used  so 
that  the  strips  may  range  evenly  from  a  black  at  one  side 
to  a  white  at  the  other,  the  corresponding  shading  of  the 
colored  paper  is  striking. 

On  a,  Helmholtz,  A,  554  ff.,  Fr.  527  ff.  (401  ff.) ;  Hering,  A,  36  ff. 
On  6,  von  Bezold,  B,  182-183  and  Plate  V.  For  what  is  perhaps  a 
related  phenomenon,  see  Briicke,  424  ff. ;  Helmholtz,  J.,  549,  Fr. 
520  (396);  Aubert,  A,  549  f. 

158.  Influence  of  Experience  in  Visual  Perception.  While 
in  the  previous  experiments  a  physiological  explanation 
seems  sufficient  for  the  facts,  psychical  action  is  not  ex- 
cluded, even  by  Hering,  from  a  considerable  share  in  sense 
perception.  In  the  following  experiments  experience  co- 
operates in  the  result. 

a.  Place  upon  the  color-mixer  a  short-pointed  star  of 
white  cardboard,  or  even  a  square;  when  in  sufficiently 
rapid  rotation,  it  appears  as  a  white  central  circle  sur- 
rounded by  a  more  or  less  transparent  ring.  While  in  this 
condition  bring  behind  it  a  broad  strip  of  black  cardboard 
of  somewhat  greater  length  than  the  diameter  of  the  star 
from  point  to  point.  As  the  edge  of  the  card  advances,  it 
can  be  seen  not  only  behind  the  transparent  ring,  but,  appar- 
ently, also  behind  the  opaque  central  circle,  and  the  portions 
of  the  latter  in  front  of  the  black  card  seem  darkened  by 
its  presence.  The  illusion  holds,  though  with  a  lightening 
instead  of  a  darkening  effect,  when  a  white  card  is  moved 
behind  a  black  star.  The  illusion  fails  by  degrees  if  the 
card  is  kept  motionless,  but  may  be  observed  to  a  certain 
extent  when  the  star  is  at  rest,  or  even  on  a  square  of  card- 


159]          SENSATIONS   OF  LIGHT  AND   COLOR.  167 

board  held  in  the  hand  while  another  is  moved  to  and  fro 
behind  it.  In  all  cases  the  latter  card  should  often  be 
wholly  withdrawn,  so  that  its  edge  can  be  clearly  seen. 

b.  Cover  a  piece  of  black  cardboard  smoothly  with  tissue 
paper,  and  notice  that  it  seems  at  first  blacker  (because  its 
color  is  well  known)  than  it  afterwards  proves  to  be  on  com- 
parison with  other  grays. 

c.  In  mixing  colors  by  reflection  (Ex.  150  a),  notice  the 
tendency  to  see  one  color  through  the  other,  instead  of  see- 
ing the  mixture  of  the  two.     This   tendency  may  be   so 
strong  at  first  as  to  interfere,  to  a  certain  extent,  with  the 
success  of  the  experiment.     See  also  Ex.  164. 

Helmholtz,  A,  312,  323  f.,  Fr.  360  (273);  Kirschmann,  E.  On  the 
difficulty  of  judging  small  differences  in  the  color  of  surfaces  that 
present  other  small  unlikenesses,  see  Bering,  E. 

SOME  PHENOMENA  OF  ROTATING  DISKS. 

159.  The  Miinsterberg-Jastrow  Phenomenon,  a.  Set  a 
black  and  white  disk,  e.g.,  that  used  in  Ex.  145  a,  in  rapid 
enough  rotation  to  give  a  uniform  gray;  pass  rapidly  before 
it  a  thin  wooden  rod  or  thick  wire,  and  notice  the  multitude 
of  shadowy  images  of  the  rod  that  appear  on  the  disk.  The 
number  of  images  is  greatest  in  the  portion  of  the  disk 
having  the  most  frequent  interchange  of  black  and  white. 

b.  Replace  the  disk  by  one  carrying  two  or  more  colors. 
Notice  the  repetition  of  the  phenomenon,  and  that  the 
colors  of  the  images  are  the  colors  (otherwise  completely 
blended)  which  the  disk  actually  carries.  The  explanation 
of  the  phenomenon  is  not  altogether  clear,  but  the  sudden 
changes  of  the  background  against  which  the  rod  is  seen 
seem  to  have  an  effect  not  unlike  that  of  a  stroboscopic  disk 
or  of  intermittent  illumination,  and  thus  show  the  rod  at 
rest  in  its  successive  positions. 

J astro w. 


168       LABORATORY  COURSE  IN  PSYCHOLOGY.       [162 

160.  Retinal  Oscillation.     Prepare  a  disk  of  black  card- 
board 25-30  cm.  in  diameter,  and  paste  upon  it  a  sector  of 
white  of  90°  extent.     Put  the  disk  in  slow  rotation  (one  turn 
a  second),  fixate  the  middle  of  the  disk,  and  notice  that  the 
retreating  edge  of  the  black  is  always  followed  by  a  narrow 
shadowy  sector  in  the  white.     Under  favorable  conditions 
more  than  one  may  be  seen.     The  retina  on  first  being  stimu- 
lated with  white,  apparently  reacts  in  the  direction  of  black 
(see  Ex.  125),  then  swings  again  toward  white,  and  so  on. 

Charpentier,  B. 

161.  Perception  of  Flicker  with  Different  Parts  of  the 
Retina.     Place  upon  the  color-mixer  a  black  and  white  disk 
in  which  the  sectors  are  complete  from  centre  to  circumfer- 
ence ;  those  used  in  Ex.  145  will  not  answer  here.     Rotate 
the  disk  at  such  a  rate  as  to  give  a  lively  flicker,  fixate  its 
centre  and  slowly  increase  the  rate.     With  care  a  point  will 
be  found  where  the  sectors  are  blended  for  the  central  parts 
of  the  retina,  but  still  flicker  for  the  periphery.     Try  also 
looking  at  one  edge  of  the  disk  while  giving  attention  to  the 
centre  or  opposite  edge.     This  is  in  accord  with  the  general 
principle  that  peripheral  after-images  are  of  shorter  duration 
than  those  of  the  retinal  centre.     Too  bright  illumination 
should  be  avoided,  for  with  intense  light  the  difference  be- 
tween the  centre  and  periphery  is  less,  or  even  quite  reversed. 

Bellarminow.  On  rotating  disks  and  their  phenomena  in  general, 
see'Helmholtz,  A,  480-501,  Fr.  445-471  (337-357). 

BINOCULAR  PHENOMENA  OF  LIGHT  AND  CoLOB.1 

162.  In  general  the  two  eyes  co-operate  to  bring  about 
a  single  visual  result,  but  the  union  of  the  impressions  upon 
the  two  retinae  is  influenced  by  a  number  of  circumstances. 

1  The  experiments  that  follow  can  all  be  made  with  the  stereoscope,  but  prac- 
tice will  enable  the  experimenter  to  combine  the  diagrams  with  free  eyes,  either 
by  crossing  the  lines  of  sight  (fixating  a  point  nearer  than  the  diagram),  or  by 
making  them  parallel  or  nearly  so  (fixating  a  point  beyond  the  diagram).  This 


163]          SENSATIONS  OF  LIGHT  AND   COLOR.  169 

a.  If  the  stimulus  to  one  eye  is  considerably  stronger 
than  that  to  the  other,  the  sensation  in  the  latter  is  in  most 
cases  totally  suppressed.     Close  one  eye  and  look  at  a  sheet 
of  white  paper  with  the  other,  letting  the  open  eye  move 
about  freely.     There  is  no  tendency  for  the  darkened  field 
of  the  closed  eye  to  assert  itself. 

b.  When,  however,  the  effect  of  the  stimulus  in  the  open 
eye  is  somewhat  weakened  by  steady  fixation,  such  a  ten- 
dency is  to  be  observed,  and  the  whole  of  the  field  of  the 
open  eye,  except  a  small  area  about  the  point  fixated,  may 
be  suppressed  from  time  to  time  by  the  dark  field  of  the 
closed  eye.     A  slight  motion  will,  however,  instantly  re- 
store the  first.     See  also  Ex.  127. 

c.  A  field  that  contains   sharply  marked  objects  or  con- 
tours will  generally  triumph  over  one  that  does  not.     Try 
combining  the  letters  below  in  such  a  way  that  the  B's  are 
superposed.     In  this  diagram  the  white  field  of  either  eye, 
which  corresponds  to  A  or  C  in  the  other  eye,  will  generally 
not  triumph  over  the  letter. 

AB      BC 

Helmholtz,  A,  Fr.  964  ff.  (767  ff.);  Hering;  P,  380-385;  Aubert, 
A,  550-553;  Wundt,  A,  3te  Aufl.,  II.,  183  ff.,  4te  Aufl.,  II.,  209  ff. 

163.  Fechner's  Paradoxical  Experiment.  Hold  close  be- 
fore one  eye  a  dark  glass,  such  as  is  used  in  protecting  the 
eyes,  or  a  piece  of  ordinary  glass  moderately  smoked  over, 
or  even  a  black  card  with  a  good-sized  pin-hole  in  it,  allow- 
ing the  other  eye  to  remain  free.  It  is  easy  to  see  that  the 

skill  the  experimenter  should  try  to  acquire.  In  these  experiments  it  is  impor- 
tant that  the  eyes  should  be  of  approximately  equal  power;  and  if  the  poorer  eye 
cannot  be  helped  with  lenses,  the  vision  of  the  other  must  be  somewhat  reduced 
by  the  interposition  of  a  sufficient  number  of  plates  of  ordinary  glass. 


170       LABORATORY  COURSE  IN  PSYCHOLOGY.      [164 

binocular  field  is  darkened  by  the  interposition  of  the  dark 
glass.  If,  however,  the  eye  behind  the  glass  is  closed,  or 
the  light  wholly  cut  off  from  it  by  holding  a  black  card  in 
front  of  the  glass,  the  field  appears  decidedly  brighter ;  that 
is  to  say,  cutting  off  a  portion  of  the  stimulus  received  by 
the  total  visual  apparatus,  has  caused  an  increased  intensity 
of  sensation.  The  experiment  fails  for  very  dark  and  very 
light  glasses.  Several  explanations  have  been  given,  but 
that  of  Aubert  (according  to  which  the  sensations  of  the 
two  retinae  blend  in  a  sort  of  average  result  when  the  dif- 
ference is  not  too  great,  but  one  wholly  suppresses  the  other 
when  the  difference  is  very  great)  seems  to  be  the  most 
satisfactory. 

Fechner,  B,  416  ff. ;  Helmholtz,  A,  Fr.  993-994  (790-791) ;  Bering, 
Q,  311  f.;  Aubert,  A,  499-503. 

164.  Rivalry.  When  the  two  retinae  are  stimulated  at 
the  same  time  separately  with  strong  light  of  different 
colors,  or  are  confronted  with  otherwise  incongruous  fields, 
i.e.,  fields  that  cannot  be  given  a  unitary  interpretation, 
there  results  a  peculiar  instability  and  irregular  alternation 
of  the  colors  over  part  or  the  whole  of  the  combined  fields 
of  vision.  This  apparent  struggle  of  the  fields  is  known 
as  Retinal  Rivalry.  Hold  close  before  one  eye  a  piece  of 
blue  glass,  before  the  other  a  piece  of  red  glass,  and  look 
toward  the  sky  or  a  brightly  lighted  uniform  wall.  The 
struggle  of  colors  will  at  once  begin.  The  same  may  be 
observed  with  a  stereoscope  when  the  usual  paired  photo- 
graphs are  replaced  by  colored  fields,  or  even  with  no  ap- 
paratus at  all,  when  both  eyes  are  closed  and  turned  toward 
a  bright  sky  and  one  of  them  is  covered  with  the  hand. 
Long  looking  generally  tends  to  quiet  the  rivalry.  Rivalry 
has  been  explained  as  due  to  fluctuations  of  attention,  and 
some  observers  find  that  it  can  be  more  or  less  controlled 
by  attention  (Helmholtz).  Fechner  discusses  the  attention 


165]          SENSATIONS   OF  LIGHT  AND   COLOR.  171 

theory,  and  finds  it  insufficient.  Von  Bezold  thinks  rivalry 
associated  with  changes  in  accommodation  which  follow 
attention.  Hering  and  others  regard  the  changes  as  of 
more  purely  physiological  origin.  See  also  Ex.  165  b. 

Helmholtz,  A,  Fr.  964  ff.  (767  ff.),  974  ff.  (775  if.);  Hering,  P, 
380-385,  Q,  308  ff.;  Aubert,  A,  550  ff.;  Wundt,  A,  3te  AufL,  II.. 
185  ff.,  4te  Aufl.,  II.,  211  ff.;  Chauveau,  C. 

165.  Prevalence  and  Rivalry  of  Contours.  By  contours 
is  here  meant  lines  of  separation  where  fields  of  one  color 
border  upon  fields  of  another  color. 

a.  Combine  stereoscopically  the  two  bars  below,  and  notice 
that  it  is  the  contours  that  suppress  the  solid  parts  of  both 
the  black  and  white.  This  figure  gives  excellent  results 
also  when  colors  are  substituted  for  the  black  and  white. 


Notice  a  similar  triumph  of  the  contours  of  the  cross  in 
the  left-hand  figure  below,  or,  better  still,  in  an  enlargement 
of  it. 


b.  Notice   the   rivalry  of   the   contours   in  all  of  these 
figures. 


172        LABORATORY   COURSE  IN  PSYCHOLOGY.      [166 

c.  The  last  two  pairs  of   diagrams  are  suitable  for  the 
study  of  the  part  played  by  attention  in  rivalry.     While 
it  is  doubtful  whether  mere  attention  to  one  field  or  the 
other  can  cause  it  to  predominate,  it  yet  seems  possible  by 
indirect  application  of  attention  to  cause  it  to  do  so.     If 
attention  is  given  to  an  examination  of  the  lines  and  small 
squares  in  the  left-hand  figure,  or  if  one  of  the  sets  of 
lines  in  the  right-hand  figure  is  counted,  both  will  appear 
to  be  somewhat  assisted  in  their  struggle  with  the  cross  or 
the  other  set  of  lines. 

d.  A  printed  page  has  a  decided  advantage.     Try  a  dia- 
gram in  which  a  printed  page  is  put  in  rivalry  with  a  field 
of  heavy  cross  lines.     The  lines  will  be  found  to  yield  to 
the  print,  at  least  at  the  point  at  which  the  reader  is  look- 
ing at  the  instant.     Two  printed  pages,  however,  become 
hopelessly  mixed ;  and  it  is  hard  to  say  how  much  of  the 
advantage,  when  a  single  one  is  used,  is  due  to  its  superior 
power  as  a  holder  of  attention,  and  how  much  to  its  excel- 
lence as  a  set  of   contours.     A  portion  of  the  power  of 
contours  is  probably  to  be  explained  by  the  mutual  intensi- 
fication of  both  the  black  and  the  white  by  contrast ;  but  a 
part  is  perhaps  due  to  a  strong  tendency,  observable  in 
other  cases  also,  for  the  eyes  (and  attention)  to  follow  lines, 
and  especially  outlines. 

Helmholtz,  A,  Fr.  964  ff.  (767  ff.);  Bering,  P,  380-385,  Q,  314: 
Wundt,  A,  3te  Aufl.,  II.,  183  ff.,  4te  Aufl.,  II.,  209  ff. 

166.  Luster.  Sheen.  When  one  of  the  rival  fields  is 
white  and  the  other  colored  (especially  when  one  is  white 
and  the  other  is  black),  there  results,  besides  the  rivalry, 
a  curious  illusion  of  shine  or  polish,  known  as  binocular 
lustre. 

a.  Examine  in  the  stereoscope  a  diagram  made  like  the 
accompanying  cut,  and  notice  the  graphite-like  shine  of  the 


167]          SENSATIONS   OF  LIGHT  AND   COLOR.  173 

pyramid.     The  explanation  seems  to  be  that  polished  sur- 
faces, which  at  some  angles  reflect  light  enough  to  look 


white,  and  at  others  appear  in  their  true  color,  have  often 
in  previous  experience  given  rise  to  such  differences  of  sen- 
sation in  the  two  eyes,  and  from  this  difference  it  is  inferred 
that  the  object  seen  in  the  diagram  is  shiny. 

b.  A  species  of  monocular  lustre  (or  transparence)  is  to 
be  observed  when  black  or  white  or  colors  are  combined  by 
means  of  the  reflection  color-mixer,  especially  when  the 
inclination  of  the  plate  is  so  changed  that  one  color  ap- 
pears to  be  reflected  in  the  surface  of  the  other,  or  to  be 
seen  through  and  behind  it.  The  experiment  works  well 
when  real  objects  are  reflected  in  the  surface  of  the  glass, 
the  reflecting  power  of  the  latter  appearing  to  be  trans- 
ferred to  the  horizontal  surface  on  the  opposite  side. 

Helmholtz,  A,  Fr.  983  ff.  (782  ff.);  Hering,  P,  576-577;  Aubert, 
A,  550  ff.;  Wundt,  A,  3te  Aufl.,  II.,  177  ff.,  183  ff.,  4te  Aufl.,  II., 
204  ff.,  209  ff. 

167.  Binocular  Color  Mixing.  The  result  of  simultane- 
ous presentation  of  different  colors  to  the  two  eyes  is  not 
always  rivalry  or  lustre.  If  the  colors  are  not  too  bright 
and  saturated,  and  the  fields  are  without  fleck  or  spot  to 


174        LABORATORY  COURSE  IN  PSYCHOLOGY.      [168 

give  one  the  predominance,  a  veritable,  though  somewhat 
unsteady,  mixture  of  the  colors  may  result. 

a.  Place  a  red  and  a  blue  glass  of  equal  transparency 
in  the  binocular  color-mixer,  and  adjust  the  side  screens  till 
the  proper  amount  of  white  light  is  mixed  in  with  that 
transmitted  from  below.     The  mixture  will  then  be  seen  on 
the  white  field  below.     Try  also  with  other  combinations  of 
glasses.     Mixtures  obtained  in  this  way  are  not  always  the 
same   in   appearance   as  the   monocular   mixtures   studied 
above,  and  some  observers  have  great  difficulty  in  getting 
them  satisfactorily.     Long  and  steady  gazing,  which  inter- 
feres with  rivalry,  favors  binocular  color  mixing. 

b.  The  same  effect  may  be  conveniently  obtained  with  a 
stereoscope,   from   which   the   middle   partition   has    been 
removed.     Try  with  equal  areas  of  dull  colors  of  little  satu- 
ration.    Bering  recommends  two  squares  of  red  and  two  of 
blue,  set  at  equal  distances  in  a  horizontal  line,  the  two  reds 
on  one  side,  the  two  blues  on  the  other.     When  the  middle 
pair  are  combined  stereoscopically,  they  show  a  mixed  color, 
while  the  unmixed  colors  can  be  seen  for  comparison  beside 
them.     He  also  suggests  the  use  of  lenses  to  prevent  sharp 
focusing  of  the  eyes  upon  the  contours,  which  interferes 
with  the  mixture.     Complementary  colors  are  said  to  be 
more  difficult  to  fuse  than  those  standing  nearer  in  the 
color  scale.     The  same  is  true  of  colors  differing  greatly  in 
brightness  ;  see  Ex.  163. 

Helmholtz,  A,  Fr.  976  ff.  (776  ff.);  Bering,  P,  591-600;  von 
Dezold,  C;  Chauveau,  A-  Aubert,  A,  550  ff.;  Wundt,  A,  3te  Aufl., 
II.,  183  ff.,  4te  Aufl.,  209  ff. 

168.  Binocular  Contrast.  The  Side- Window  Experiment. 
Stand  so  that  the  light  from  the  window  falls  sidewise  into 
one  eye,  but  not  at  all  into  the  other.  Place  in  a  convenient 
position  for  observation  a  strip  of  white  paper  on  a  black 
surface.  The  paper  when  looked  at  with  both  eyes  appears 


169]  SENSATIONS   OF  LIGHT  AND   COLOR.  175 

perfectly  colorless.  On  looking  now  at  a  point  nearer  than 
the  strip  of  paper  (e.g.,  at  the  finger  held  up  before  the  face), 
double  images  of  the  strip  will  be  seen.  The  two  images  will 
be  different  in  brightness  and  slightly  tinged  with  comple- 
mentary colors.  The  image  belonging  to  the  eye  next  the 
window  (which  may  be  recognized  by  its  disappearance 
when  that  eye  is  closed)  will  appear  tinged  with  a  faint 
blue  or  blue-green  color,  the  other  with  a  very  faint  red  or 
yellow.  The  light  that  enters  the  eye  through  the  sclerotic 
is  tinged  reddish  yellow,  and  makes  the  eye  less  responsive 
to  that  color  ;  the  white  of  the  paper  strip  therefore  appears 
bluish.  It  appears  darker  partly  for  a  similar  reason,  and 
perhaps  also,  as  Fechner  suggests,  because  it  lies  in  a  field 
which,  for  the  eye  in  question,  is  generally  bright.  The 
reddish  color  of  the  other  eye's  image  of  the  strip  is  ex- 
plained as  due  to  contrast  with  the  first ,  but  whether  this 
contrast  color  is  a  psychical  matter,  or  whether  it  is  to  be 
explained  by  the  action  of  the  stimulus  in  the  first  eye 
upon  the  second,  as  there  seems  some  reason  to  think,  is  as 
yet  uncertain.  Its  greater  brightness  is  probably  due  to 
the  fresher  condition  of  the  eye  to  which  it  belongs,  and  to 
contrast  with  its  less  brilliant  field.  The  same  thing  is 
often  to  be  noticed  when  reading  with  the  lamp  at  one  side, 
or  even  when  one  eye  has  been  closed  for  a  short  time 
while  the  other  has  been  open.  The  double  images  are 
in  no  wise  essential ;  simple  alternate  winking  will  show 
decided  differences  in  the  condition  of  the  two  eyes. 

Fechner,  B,  511  ff.;  Briicke,  420  ff.;  Bering,  P,  600-601;  Helm- 
holtz,  A,  Fr.  987  ff.  (785  ff.);  Chauveau,  B;  Titchener,  Wundt,  A, 
3te  Aufl.,  II.,  183  ff.,  4te  Aufl.,  II.,  209  ff. 

169.  Binocular  After-images.  Lay  a  bit  of  orange-colored 
paper  on  a  dark  ground,  and  provide  two  white  cards.  Hold 
one  of  the  cards  close  to  the  left  eye,  but  a  little  to  one 
side,  so  as  not  to  hide  the  bit  of  paper.  Hold  the  other 


176        LABORATORY  COURSE  IN  PSYCHOLOGY.      [l69 

eight  or  ten  inches  from  the  right  eye  in  such  a  way  as  to 
hide  the  paper.  Look  at  the  paper  for  a  few  seconds  with 
the  left  eye,  then  bring  the  card  before  it.  A  faint,  washy, 
orange-colored  positive  after-image  will  appear  on  the  card 
before  the  right  eye.  The  image  is  by  no  means  easy  to 
observe.  It  is  supposed  to  belong  to  the  right  eye's  half 
of  the  visual  apparatus,  possibly  to  the  central,  i.e.,  cerebral, 
part. 

Ebbinghaus,  C ;  Chauveau,  B  ;  Titchener. 


BIBLIOGRAPHY. 

ABNEY:  A.  On  the  Examination  for  Colour  of  Cases  of  Tobacco 
Scotoma  and  of  Abnormal  Colour  Blindness,  Proc.  Roy.  Soc., 
XLIX.,  1891,  491-508. 

B.  On  the  Limit  of  Visibility  of  the  different  Rays  of  the  Spec- 
trum, ibid.,  XLIX.,  1891,  509-518. 

C.  The  Sensitiveness  of  the  Eye  to  Light  and  Colour.     Nature, 
XL VII.,  1892-93,  538-542. 

ABNEY    AND    FESTING:    Colour    Photometry,    iii.,    Phil.    Trans., 

CLXXXIIL,  1892,  A,  531-565. 
ALBERT:  Ueber  die  Aenderung  des  Farbentones  von  Spectralfarben 

und  Pigmenten  bei  abnehmen  der  Lichtstarke,   Wiedemanrfs 

Annalen,  XVI.,  1882,  129-160. 
AUBEBT:  A  and  B.     Works  cited  with  same  letters  in  bibliography 

of  Chap.  V. 
BELLARMINOW:  Ueber  intermittirende  Netzhautreizung,  von  Graefe's 

Archiv,  XXXV.,  1889,  i.,  25-49. 

BENSON:  Manual  of  the  Science  of  Colour,  London,  1871. 
VON  BEZOLD:  A.     Ueber  das  Gesetz  der  Farbenmischung  und  die 

physiologischen    Grundfarben,    Poggendorff"1  s    Annalen,  CL., 

1873,  71-93,  221-247. 
B.   The  Theory  of  Color  in  its  Relation  to  Art  and  Art  Industry, 

Boston,  1876. 


SENSATIONS  OF  LIGHT  AND   COLOR.  177 

C.    Ueber  binoculare    Farbenmischung,  Poggendorff's  Annalen, 
Jubelband  [1874],  585-590.     Cites  earlier  literature. 

BRODHUN:  A.  Ueber  die  Empfindlichkeit  des  griinblinden  und  des 
normalen  Auges  gegen  Farbenanderung  im  Spektrum,  Zeit- 
schriftfur  Psychologie,  III.,  1892,  97-107. 

B.  Die    Giiltigkeit  des   Newton' schen    Farbenmischungsgesetzes 
bei  dem  sog.  griinblinden  Farbensystem,  ibid.,  V.,  1893,  323- 
334.     Cites  literature. 

BJBUCKE:  Untersuchungen  iiber  subjective  Farben,  PoggendorJFs 
Annalen,  LXXXIV.,  1851,  418-447. 

CHARPENTIER  :  A  and  B.  Work  cited  with  same  letters  in  bibliog- 
raphy of  Chap.  V. 

C.  Sur  le  retard  dans  la  perception  des  divers  rayons  spectraux, 
Comptes  rendus,  CXIV.,  1892,  1423-1426. 

CHAUVEAU:  A.  Sur  la  fusion  des  sensations  chromatiques  per- 
cues  isolement  par  chacun  des  deux  yeux,  Comptes  rendus, 
CXIIL,  1891,  358-362. 

B.  Sur  les  sensations  chromatiques  exercitees  dans  1'un  des  deux 
yeux  par  la  lumiere  coloree  qui  eclaire  la  retine  de  1'autre  oeil, 
ibid.,  394-398. 

C.  Sur  la  theorie  de  1'antagonisme  des  champs  visuels,  ibid.,  439- 
442. 

CHEVREUL:  The  Principles  of  Harmony  and  Contrast  of  Colours, 
London,  1859. 

DELABARRE:  Colored  Shadows,  American  Journal  of  Psychology, 
II.,  1888-89,  636-643. 

BONDERS:  A.  Ueber  Farbensysteme,  von  Graefe's  Archiv,  XXVIL, 
1881,  i.,  155-223. 

B.  Noch  einmal  die  Farbensysteme,  ibid.,  XXX.,  1884,  i.,  15-90. 

C.  Farbengleichungen,  DuBois-Reymond's  Archiv,  1884,  518-552. 
DOVE:   Versuche    iiber   subjective    Complementarfarben,    Poggen- 

dorff's  Annalen,  XLV.,  1838,  158-162. 

EBBINGHATJS:  A.  Theorie  des  Farbensehens,  Zeitschrift  fur  Psy- 
chologie,  V.,  1893,  145-238.  Full  statement  of  matter  pre- 
sented in  outline  before  the  Psychological  Congress  in  London, 
1892,  Proceedings,  101-103. 


178        LABORATORY   COURSE  IN  PSYCHOLOGY. 

B.  Die  Gesetzmassigkeit  des  Helligkeitscontrastes,  Sitz.-ber.  der 
Akademie  zu  Berlin,  1.,  Dec.,  1887. 

C.  Ueber  Nachbilder  im  binocularen  Sehen  und  die  binocularen 
Farbenerscheinungen    iiberhaupt,    Pfluger's    Archiv,   XLVL, 
1890,  498-508. 

EBERT:   Ueber  den  Einfluss  der  Schwellenwerthe  der  Lichtempfin- 

dung  auf  den  Charakter  der  Spectra,   Wiedemanris  Annalen, 

XXXIII.,  1888,  136-155. 
EXNER:  A.     Bemerkungen  iiber  intermittirende  Netzhautreizung, 

Pfliiger's  Archiv,  III.,  1870,  214-240. 
B.   Ueber  eine  neue  Urtheilstauschung  im  Gebiete  des  Gesichts- 

sinnes,  ibid.,  XXXVII.,  1885,520-522,  (this  part  also  in  Biol. 

Centralbl.,  VI.)  ;  XL.,  1887,  323-330. 

FECHNER:  A.  Ueber  eine  Scheibe  zur  Erzeugung  subjectiver  Far- 
ben,  Poggendorff's  Annalen,  XLV.,  1838,  227-232. 
B.   Ueber  einige  Verhaltnisse  des  Binocularen  Sehens,  Abhl.  d. 
k.  sacks.  Ges.  d.  Wiss.,  VII.,  1860,  339-564. 

FICK,  A. :  A.     Zur  Theorie  des  Farbensinnes  bei  indirektem  Sehen, 

Pfliiger's  Archiv,  XL VII.,  1890,  274-285. 
B.    Work  cited  with  same  letter  in  Chap.  V. 

FICK,  A.   E. :  A.    Eine   Notiz   iiber  Farbenempfindung,  Pfluger's 

Archiv,  XVII. ,  1878,  152-153. 

B.   Studien   iiber  Licht-  und   Farbenempfindung,   ibid.,  XLIIL, 
1888,  441-501. 

FRANKLIN,  Christine  Ladd:  A.  Eine  neue  Theorie  der  Lichtem- 
pfindung,  Zeitschriftfur  Psychologic,  IV.,  1892,  211-221.  Ab- 
stracts of  this  paper  may  be  found  in  Proc.  Congr.  Exper.  Psy., 
London,  1892;  Johns  Hopkins  University  Circulars,  XII.,  No. 
106,  June,  1893,  108-110;  Science,  XXII.,  July  14,  1893,  18-19. 
B.  On  Theories  of  Light  Sensation,  Mind,  Ser.  2,  II.,  1893,  473- 
489. 

HELMHOLTZ:  A.     Work  cited  with  same  letter  in  bibliography  of 
Chap.  V. 

B.  Popular  Scientific  Lectures,  First  Series,  New  York,  1885. 

C.  Versuch  einer  erweiterten  Anwendung  des  Fechnerschen  Ge- 
setzes  im  Farbensystem,  Zeitschrift  fur  Psychologic,  II.,  1892, 
1-30. 


SENSATIONS   OF  LIGHT  AND  COLOR.  179 

J>.  Yersucli  das  psych ophysische  Gesetz  auf  die  Farbenunter- 
schiede  trichromatischer  Augen  anzuwenden,  ibid.,  III.,  1891, 
1-20. 

E.     Kiirzeste  Linien  im  Farbensystem,  ibid.,  108-122.    An  extract 

from  Sitz.-ber.  der  Akademie  zu  Berlin,  17,  December,  1891. 
HERiNG:1  A.     Zur  Lehre  vom  Lichtsinne,  Wien,  1878.    Keprint  of 
six  communications  to  the  Vienna  Academy,  1872-74.     For  an 
extended  abstract  of  this  work,  made  by  Dr.  William  Pole,  see 
Nature,  XX.,  1879,  611-613,  637-639;  XXI.,  1879-80,  14-17. 

B.  Zur  Erklarung  der  Farbenblindheit  aus  der  Theorie  der  Gegen- 
farben,  Prag,    1880.     Reprint  from  Lotos,  Neue  Folge,  I.,  1880. 

C.  Ueber  individuelle  Verschiedenheiten  des  Farbensinns,  Lotos, 
Neue  Folge,  VI.,  1885. 

D.  Beleuchtung  eines  Angriffes  auf  die  Theorie  der  Gegenfarben, 
Pfluger1*  Archiv,  XLL,  1887,  29-46. 

E.  Ueber  die  Theorie  des  simultanen  Contrastes  von  Helmholtz, 
ibid.,  XL.,  1886-87,  172-191    (Die   farbigen    Schatten);    XLL, 
1887,   1-29   (Der  Contrastversuch  von  H.  Meyer  und  die  Ver- 
suche  am  Farbenkreisel) ;  358-367  (Der  Spiegelcontrastversuch) ; 
XLIIL,  1888,  1-21  (Die  subjective  ,,  Trennung  des  Lichtes  in 
zwei  complementare  Portionen  "). 

F.  Ueber  die  von  v.  Kries  wider  die  Theorie  der  Gegenfarben 
erhobenen  Einwande,  ibid.,  XLIL,  1888,  488-506;  XLIIL,  1888, 
264-288,  329-346. 

G.  Ueber  die  Hypothesen  zur  Erklarung  der  peripheren  Farben- 
blindheit, v.  Graefe's  Archiv,  XXXV.,  1889,  iv.,  63-83;  XXX VI. , 
1890,  i.,  264. 

H.   Zur  Diagnostik  der  Farbenblindheit,  ibid.,  XXXVI. ,  1890,  i., 

217-233. 
7.   Die  Untersuchung  einseitiger  Storungen  des  Farbensinnes  mit- 

tels  binocularer  Farbengleichungen,  ibid.,  XXXVI. ,  1890,  iii., 

1-23. 
J.   Beitrag  zur  Lehre  vom  Simultankontrast,  Zeitschrift  fur  Psy- 

chologie,  L,  1890,  18-28. 

1  Herlng's  work  upon  color  has  not  yet  been  gathered  into  one  consecutive 
whole.  It  has  seemed  well,  therefore,  to  insert  here,  in  addition  to  the  titles  of 
papers  bearing  directly  on  the  experiments  of  Chap.  VI.,  such  other  titles  on 
light  and  color  as  came  to  hand. 


180       LAtiOllATonY  COU11SE  IN  PSYCHOLOGY. 

K.   Eine    Methode    zur    Beobachtung    des    Simultancontrastes, 

Pfliiger's  Archiv,  XLVII.,  1890,  236-242. 
L.    Priif ung  der  sogenannten  Farbendreiecke  mit  Hiilfe  des  Farben- 

sinns  excentrischer  Netzhautstellen,  ibid.,  XLVII.,  1890,  417- 

438. 
M.   Ueber   Newton's    Gesetz    der  Farbenmischung,  Prag,    1887. 

Reprint  from  Lotos,  VII.,  1887. 
N.   Untersuchung  eines  total  Farbenblinden,  Pfluger^s  Archiv, 

XLIX.,  1891,  563-608. 
O.   Eine  Vorrichtung  zur  Farbenmischung,  zur  Diagnose  der  Far- 

benblindheit  und  zur  Untersuchung  der  Contrasterscheinungen, 

Pfluger's  Archiv,  XLII.,  1888,  119-144. 
P.   Work  cited  with  reference  letter  A  in  the  bibliography  of 

Chap.  V. 
Q.   Work  cited  with  reference  letter  B  in   the  bibliography  of 

Chap.  V. 
It.   Ueber    Holmgren's    vermeintlichen    Nachweis   der    Elemen- 

tarempfindungen    des    Gesichtssinns,    Pfluger^s   Archiv,    XL., 

1887,  1-20. 
S.   Kritik  einer  Abhandlung  von  Bonders,  Lotos,  Neue  Folge,  II., 

Prag,  1882. 
T.   Ueber    Sigmund    Exner's   neue   Urtheilstauschung  auf   dem 

Gebiete  des  Gesichtsinnes,  Pfliiger's  Archiv,   XXXIX.,  1886, 

159-170. 
V.    Ueber  den   Begriff  ,,  Urtheilstauschung "   in   der  physiologi- 

schen  Optik  und  iiber  die  Wahrnehmung  simultaner  und  suc- 

cessiver  Helligkeitsuntersjchiede,  ibid.,  XLI.,  1887,  91-106. 

HESS:  A.  Ueber  den  Farbensinn  bei  indirectem  Sehen,  v.  Graefe's 
Archiv,  XXXV.,  1889,  iv.,  1-62. 

B.  Untersuchung  eines  Falles  von  halbseitiger  Farbensinnsstorung 
am  linken  Auge,  ibid.,  XXXVI.,  1890,  iii.,  24-36. 

C.  Ueber  die  Tonanderungen  der  Spektralfarben  durch  Ermiidung 
der  Netzhaut  mit  homogenem  Lichte,  ibid.,  XXXVI.,  1890,  L, 
1-32. 

HILLEBBAND  :  Work  cited  in  bibliography  of  Chap.  V. 

HOLMGBEN  :  Color-blindness  in  its  Relation  to  Accidents  by  Rail  and 
Sea.  Translation  by  M.  L.  Duncan,  Smithsonian  Report,  1877, 
131-195. 


SENSATIONS  OF  LIGHT  AND   COLOR  181 

JASTROW:  A  Novel  Optical  Illusion,  American  Journal  of  Psy- 
chology, IV.,  1891-92,  201-208. 

JEFFRIES:  A.  Color-blindness,  its  Dangers  and  its  Detection,  Bos- 
ton, 1879.  This  work  contains  a  seventeen-page  bibliography 
on  color-blindness  and  kindred  topics. 

B.   Color-blindness,  Article  in  the  Reference  Handbook  of  the 
Medical  Sciences,  New  York,  1886,  II.,  241. 

KIRSCHMANN:  A.  Beitrage  zur  Kenntniss  der  Farbenblindheit, 
WundV s  PMlos.  Studien,  VIII.,  1892-93,  173-230,  407-430. 

B.  Ueber  die  Helligkeitsempfindung  ira  indirecten  Sehen,  ibid., 
V.,  1889,  447-497. 

C.  Die  Farbenempfindung  im  indirecten  Sehen,  Erste  Mittheilung, 
ibid.,  VIII.,  1892-93,  592-614. 

D.  Ueber  die  quantitativen  Verhiiltnisse  des  simultanen  Hellig- 
keits-  und  Farben-Contrastes,  ibid.,  VI.,  1890,  417-491. 

E.  Some  Effects  of  Contrast,  American  Journal  of  Psychology, 
IV.,  1892,  542-557. 

KONIG:  A.  Ueber  den  Helligkeitswert  der  Spektralfarben  bei 
verschiedener  absoluter  Intensitat,  Beitrage  zur  Psychologic 
und  Physiologic  der  Sinnesorgane  (Helmholtz  Festgruss),  Ham- 
burg und  Leipzig,  1891,  311-388. 

B.  The   Modern   Development   of    Thomas  Young's   Theory  of 
Colour  Vision,   Report   of  British  Association,   Birmingham 
Meeting,  1886,  431-439. 

C.  Zur  Kenntniss  dichromatischer  Farbensysteme,  Wiedemann's 
Annalen,  XXII.,  1884,  567-578. 

KONIG  UND  DIETERICI:  A.  Die  Grundempfindungen  in  normalen 
und  anomalen  Farbensystemen  und  ihre  Intensitiitsverteilung 
im  Spektrum,  Zeitschrift  fur  Psychologic,  IV.,  1892,  241-347. 
B.  Ueber  die  Empfindlichkeit  des  normalen  Auges  fur  Wellen- 
langenunterschiede  des  Lichtes,  Wiedemann's  Annalen,  XXII., 
1884,  579-589. 

VON  KRIES  :  Die  Gesichtsempfindungen  und  ihre  Analyse,  Du  Bois- 
Reymond's   Archiv,  1882,    Supplement-Band,  1-178.     A  care- 
ful summary  and  discussion  of  the  whole  subject. 
LEHMANN  :  Ueber  die  Anwendung  der  Methode  der  mittleren  Abstu- 
fungen  auf  den    Lichtsinn;  die  quantitative   Bestimmung  des 
Lichtcontrastes,  WundVs  Philos.  Studien,  III.,  1886,  516-528. 


182        LABORATORY  COURSE  IN  PSYCHOLOGY 

MAXWELL  :  A.  On  the  Theory  of  Compound  Colours,  and  the  Relation 

of  the  Colours  of  the  Spectrum,  Phil.  Trans.,  CL.,  1860,  57-84. 
B.    On  Colour  Vision,  Proc.  Royal  Institution  of  Great  Britain,  VI. 

These  two  papers  are  also  to  be  found  in  Maxwell's  Scientific 

Papers,  Cambridge,  1890,  I.,  410-440,  II.,  267-280. 
MAYER:  Studies  of  the  phenomena  of  Simultaneous  Contrast-Color; 

and  on  a  Photometer  for  measuring  the  intensities  of  Lights  of 

different  colors,  American  Journal  of  Science,  Ser.  3,  XL VI., 

1893,  1-22;  also  Phil.  Mag.,  Ser.  5,  XXXVI.,  1893,  153-175. 
MEYER:  Ueber  Contrast-  oder  Complementarfarben,  Poggendorff's 

Annalen,  XCV.,  1855,  170-171;   also  Phil.  Mag.,  Ser.  4,  IX., 

Jan.-June,  1855,  547. 
NICHOLS:  A.  On  the  Sensitiveness  of  the  Eye  to  Colors  of  a  Low 

Degree  of  Saturation,    American  Journal  of  Science,  Ser.   3, 

XXX.,  1885,  37-41. 
B.   Duration  of  Color  Impressions   upon   the   Retina,  American 

Journal  of  Science,  Ser.  3,  XXVIII.,  1884,  243-252. 
PACE:    Zur  Frage  der  Schwankungen   der  Aufmerksamkeit   nach 

Versuchen    mit   der  Masson'schen    Scheibe,    Wundfs   Philos. 

Studien,  VIII.,  1892-93,  388-402. 

PEIRCE,  B.  O.,  JR.  :  On  the  Sensitiveness  of  the  Eye  to  Slight  Differ- 
ences of  Color,  American  Journal  of  Science,  Ser.  3,  XXVI., 
1883,  299-302. 
PEIRCE,  C.  S. :  Note  on  the  Sensation  of  Color,  American  Journal 

of  Science,  Ser.  3,  XIII.,  1877,  247-251. 

PLATEAU:  Betrachtungen  iiber  ein  von  Hrn.  Talbot  vorgeschla- 
genes  photometrisches  Princip,Poggendorff's  Annalen,  XXXV., 
1835,  457-468. 

POLE  :  Further  Data  on  Colour-Blindness,  Phil.  Mag.,  Ser.  5,  XXXIV. , 
1892,  100-114,  439-443,  XXXV.,  1893,52-62,  XXXVI.,  1893, 
188-195. 

PREYER:  Work  cited  in  bibliography  of  Chap.  I. 
RAYLEIGH:  A.   Experiments  on  Colour,  Nature,  XXV.,  1881-82, 

64-66. 

B.  Rayleigh  and  others:  Report  of  the  [Royal  Society's]  Commit- 
tee on  Colour- Vision,  Proc.  Roy.  Soc.,  LI.,  No.  311,  July  19, 
1892,  281-396. 


SENSATIONS   OF  LIGHT   AND   COLOR.  183 

ROOD:  A.   Students'  Textbook  of  Color,  New  York,  1881. 

B.    On  a  new  Theory  of  Light,  proposed  by  John  Smith,  M.A., 

American  Journal  of  Science,  Ser.  2,  XXX.,  1860,  182-186. 
SCHUSTER:   Experiments  with  Lord   Rayleigh's  Colour  Box,  Proc. 

Roy.  Soc.,  XLVIIL,  1890,  140-149. 
TALBOT:   Experiments  on  Light,  Phil.  Mag.,  Ser.  3,  V.,  July-Dec., 

1834,  321-334,  especially,  327-334. 
TITCHENER:   Ueber     binoculare    Wirkungen     monocularer    Reize, 

WundVs  Philos.  Studien,  VIII.,  1892-93,  231-310.     Cites  litera- 
ture. 
WUNDT:  A.   Work  cited  with    same   letter  in  the  bibliography  of 

Chap.  V. 
B.   Die  Empiindung  des  Lichts  und  der  Farben,  WundVs  Philos. 

Studien,  IV.,  1888,  311-389. 

For  further  bibliographical  references,  see  the  works  of  Helmholtz 
and  Aubert  and  the  following  by  Plateau :  Bibliographic  analytique 
des  principaux  phenomenes  subjectifs  de  la  vision,  depuis  les  temps 
anciens  jusqu'a  la  fin  du  XVIII.  siecle;  suivie  d'une  bibliographic 
simple  pour  la  partie  ecoulee  du  siecle  actuel.  Mem.  cour.  de  1'Acad. 
R.  de  Belgique ;  Bruxelles,  1876-77. 


184       LABORATORY   COURSE  IN  PSYCHOLOGY. 


CHAPTER   VII. 
Visual  Perception  of  Space  and  Motion. 

THE  question  of  visual  space  perception  is  one  of  th<r 
oldest  and  most  actively  discussed  in  all  physiological  psy- 
chology.    A  complete  treatment  involves  arguments  from 
surgery,  pathology,  and  other  sources  outside  the  possibi1' 
ties  of  the  laboratory ;  and  even  then  it  is  difficult,  if  noi 
impossible,  to  establish  one  theory  of  it  surely,  as  against 
all  others.     Apart  from  the  question  of  original  sensation^ 
there  is,  however,  a  certain  degree  of  harmony,  and  it  is  tl. 
commonly  accepted  facts  that  this  chapter  aims  to  gather 
up.     The  discussion  of  the  ultimate  matters  may  be  f 
lowed  in  the  works  of  Helmholtz,  Hering,  Stumpf,  Jair 
Wundt,  and  others.     For  the  facts  in  general,  see  Helm- 
holtz, Hering,  Aubert,  Wundt,  James,  and  Le  Conte ; 
special   facts,   see   special   references    given  below.      T 
subject  is  also  treated  in  the  standard  physiologies,  Bern 
stein's  Five  Senses,  McKendrick  and   Snodgrass's  Physir 
ology  of  the  Senses,  and  other  books  of  the  same  kind. 

The  ordinary  seeing  of  space  rests  on  the  retinal  aiv> 
kinaesthetic  sensations  of  both  eyes,  and  in  every  norm 
act  of  vision  any  or  all  these  sensations  may  be  influen- 
tial. For  the  sake  of  simplicity,  however,  it  is  necessary 
to  separate  them,  and  to  treat  now  one  and  now  another. 
The  topics  will  be  taken  up  in  the  following  order  :  Monoc- 
ular Perception  of  Space  (including  cases  where  both  eyes 
are  used,  but  the  conditions  are  not  essentially  influenced 
thereby)  ;  Geometrical  Illusions  ;  Equivocal  Figures  ;  Binoc- 


170]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         185 

ular  Perception  of  Space;  Visual  Perception  of  Motion; 
and  Visual  Symmetry. 

MONOCULAR  PERCEPTION  OF  SPACE. 

170.  The  Outward  Reference  of  Visual  Perceptions. 
Outward  reference  of  visual  perceptions  probably  comes 
about  through  their  co-ordination  with  the  perceptions  of 
other  senses,  especially  the  tactual  and  kinaesthetic,  but  the 

natter  is  too  complex  for  direct  experiment.  It  is  easy, 
however,  to  study  the  relation  of  the  retinal  image  to  the 
outer  objects  that  produce  it.  Considered  physically,  the 

nage  is  reversed.  (Cf.  the  experiment  on  the  rabbit's  eye, 
Ex.  104,  and  those  on  Purkinje's  vessel  figures  and  phos- 
phenes,  Exs.  Ill  and  119)  :  it  can  be  shown  also  in  the 

Allowing  experiment  with  retinal  shadows  ;  but  in  all  cases 

^must  be  kept  clearly  in  mind  that  retinal  phenomena  are 
never  perceived  as  such,  and  especially  that  retinal  sensa- 
ns  are  not  first  given  a  location  in  the  eye,  and  later 
nsf erred  outward.  (?<JAT£- 

**a.    Ketinal  Shadows  ;  Le  (it's  Experiment.    Hold  a  pin, 

ul  upward,  as  close  as  possible  before  the  pupil,  and,  an 

Ji  or  two  in  front  of  the  pin,  a  card 

pierced  with  a  pin-hole.     Move  the  pin 

about  till  it  comes  into  exact  line  with 

:he  hole,  when  there  will  be  seen  in  the 
;'frcle  of  diffusion  representing  the  hole 

>£ shadowy  inverted  image  of  the  pin- 
riead,  somewhat  as  appears  in  the  accom- 
panying  cut.  The  rays  of  light  from  the  pin-hole  are  too 
divergent  to  be  brought  to  a  focus  on  the  retina,  but  enter  the 
eye  in  a  favorable  state  for  casting  a  shadow.  The  shadow 
on  the  retina  is  erect,  like  the  pin  that  casts  it,  but  is  per- 
ceived as  inverted.  Observe  at  the  same  time  the  still 
more  blurred,  erect  image  of  the  pin  through  which  the 


186      LABORATORY  COURSE  IN  PSYCHOLOGY.      [171 

other  things  are  seen.  This  is  not  a  shadow,  but  an  image 
(really  a  blur  of  diffusion  circles)  formed  in  the  ordinary 
way  by  light  reflected  from  the  surface  of  the  pin.  When 
several  pin-holes  are  used  (three  at  the  points  of  an  eighth 
of  an  inch  triangle,  for  example),  an  equal  number  of  shad- 
ows will  be  seen. 

The  casting  of  the  shadow  can  easily  be  illustrated  with 
a  candle  and  a  double  convex  lens.  Set  the  lens  a  foot  or 
two  from  the  candle,  and  hold  a  card  on  the  opposite  side 
of  the  lens,  too  near  for  the  formation  of  an  image ;  then 
introduce  a  finger  or  pencil  close  before  the  lens  on  the  side 
toward  the  light,  and  observe  the  erect  shadow  on  the  card. 

b.  After-images  and  other  retinal  phenomena  often  ap- 
pear to  be  conformed  to  the  surface  of  objects  upon  which 
they  are  projected.  This  has  been  made  a  subject  of  ex- 
periment in  Ex.  124  b,  and  another  example  was  found  in 
the  distortion  of  the  after-image  cross  in  Ex.  131  b.  If 
the  surface  is  complicated  and  the  after-image  strong,  this 
conformation  is  not  apt  to  occur,  the  image  appearing  in- 
stead to  float  before  the  surface.  Get  a  monocular  after- 
image of  a  narrow  slit  in  the  window  shutters,  or  of  a 
polished  steel  rod  set  up  in  the  sunlight,  and  project  it 
into  one  corner  of  the  room.  The  after-image,  especially 
when  somewhat  faded,  can  be  made  to  lie  part  on  one  wall 
and  part  on  another,  and  thus  to  appear  bent  and  distorted. 

On  monocular  projection  in  general,  Helmholtz,  A,  G.  758  if. 
Fr.  780  ff.  (613  ff.);  Aubert,  A,  600  ff.,  619  f.  ;  Hering,  A,  572  ff. 
On  a,  Le  Conte,  B  ;  Wallenberg  ;  Laqueur.  On  6,  Thiery,  315  f.  ; 
Scharwin  and  Novizki. 

171.  Monocular  Perception  of  Directions  from  the  Eye. 
The  perception  of  direction  is  ordinarily  binocular,  and 
the  centre  to  which  directions  are  referred  lies  between  the 
eyes,  even  when  one  is  closed.  (See  Ex.  207.)  Binocular 


171]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         187 

perception  must,  however,  rest  on  a  perception  of  the  rela- 
tive direction  of  points  in  the  monocular  field,  and  this 
will  be  considered  in  the  next  few  experiments. 

Two  luminous  points  appear  to  have  the  same  direction 
when  one  is  exactly  covered  by  the  other ;  or,  to  state  the 
matter  in  retinal  terms,  when  the  image  of  the  one  for 
which  the  eye  is  accommodated  lies  in  the  centre  of  the 
circle  of  diffusion  of  the  one  for  which  the  eye  is  not  ac- 
commodated ;  or,  if  both  appear  in  diffusion  circles,  when 
the  centres  of  these  circles  coincide.  The  lines  drawn 
through  points  in  this  relation  are  known  as  Sighting 
Lines.  When  prolonged  toward  the  eye,  they  meet  in  the 
centre  of  the  pupil,  or,  rather,  in  the  centre  of  the  image  of 
the  pupil  formed  by  the  cornea,  about  0.6  mm.  forward  of 
the  true  position  of  the  pupil,  and  3  mm.  from  the  summit 
of  the  cornea.1  The  sighting  line  which  coincides  with  the 
line  of  vision  is  the  primary  sighting  line. 

The  Parallax  of  Indirect  Vision.  The  position  of  the 
common  point  of  sighting  lines  is  found  by  inference  from 
the  optical  structure  of  the  eye.  To  make  a  sure  empirical 
determination  would  be  laborious,  but  it  is  easy  to  show 

1  These  lines  (  Visirlinien,  Lignes  de  viset)  might  well  be  called  "  lines  of  di- 
rection," had  not  this  name  been  already  given  to  another  set  of  lines,  those, 
namely,  which  are  drawn  from  the  points  of  external  objects  to  the  correspond- 
ing points  of  the  retinal  image.  These  have  been  mentioned  in  Exs.  106  and  117 ; 
and  they  give,  with  certain  limitations,  the  directions  in  which  objects  appear 
when  the  eye  is  exactly  accommodated  for  them.  Their  point  of  intersection  is 
about  7  mm.  from  the  summit  of  the  cornea.  They  are  important  for  physio- 
logical optics,  but  for  the  psychology  of  the  perception  of  direction  are  less 
important  than  the  sighting  lines,  though  for  remote  points,  and  for  points  near 
the  fixation  point,  the  difference  between  the  two  sets  of  lines  is  very  slight. 
For  points  remote  from  the  fixation  point,  for  reasons  to  be  given  later  (Ex.  172), 
neither  set  gives  exactly  the  direction  in  which  objects  are  seen. 

Kirschmann  (p.  474  ff.)  has  called  attention  to  an  error  into  which  the  un- 
wary are  apt  to  be  led  by  the  term  "  crossing  point  of  sighting  lines  ;  "  namely, 
that  these  lines,  when  extended  to  the  retina,  give  the  position  of  the  centres  of 
the  circles  of  diffusion.  A  more  appropriate  name  for  the  point  in  question 
would  be  the  common  point  of  sighting  lines.  For  diagrams  correctly  drawn 
in  this  particular  see  Kirschmann,  Figs.  4-6. 


188      LABORATORY  COURSE  IN  PSYCHOLOGY.     [172 

that  the  point  is  considerably  in  front  of  the  centre  of 
rotation  of  the  eye  (about  10.6  mm.).  The  difference  be- 
tween the  visual  angle  of  any  point  (that  is,  the  angle 
made  by  the  sighting  line  passed  through  that  point  and 
the  primary  sighting  line)  and  its  rotational  angle  (that  is, 
the  angle  through  which  the  eye  must  be  turned  to  fixate 
that  point)  is  the  parallax  of  indirect  vision. 

Place  a  candle  at  a  distance  of  a  foot  or  a  foot  and  a 
half  from  the  eye.  Look  toward  the  flame  with  a  single 
eye,  but  hold  close  before  the  eye  a  pencil  or  narrow  strip 
of  cardboard.  So  long  as  the  eye  looks  straight  forward, 
the  flame  is  entirely  hidden  by  the  pencil.  When,  how- 
ever, the  eye  is  turned  strongly  to  either  side,  the  flame 
instantly  appears  on  the  side  toward  which  the  eye  has 
been  turned.  Such  differences  in  apparent  direction  will 
be  large  if  one  of  the  points  is  near  and  the  movements  of 
the  eye  are  extensive,  but  small  when  both  points  are  dis- 
tant or  the  movements  small.  Similar  shiftings  are  caused 
by  changes  of  accommodation.  Dr.  Kirschmann  considers 
such  changes  an  important  element  in  the  monocular  per- 
ception of  distance. 

The  explanation  of  the  parallax  will  readily  appear  from 
the  following  diagrams,  in  which  p  represents  the  pencil, 
/  the  flame,  s  the  common  point  of  sighting  lines,  and  the 
dot  on  sn  the  centre  of  rotation.  The  lines  radiating  from 
s  are  sighting  lines,  sa  being  the  principal  one,  which  is 
practically  coincident  with  the  line  of  sight. 

Helmholtz,  A,  G.  680,  727  ff.  Fr.,  692  (539),  745  S.  (583  if.);  Au- 
bert,  A,  461;  Kirschmann. 

172.  Relative  Directions  in  the  Monocular  Field  of  Vis- 
ion.1 


1  For  direction  of  the  apparent  vertical,  which  might  alsolbe  included  here, 
see  Ex.  209  b. 


172]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         189 

a.  Lines  that  Appear  Straight  in  Indirect  Vision.  Lay 
a  large  sheet  of  paper  on  the  table,  and  mark  a  fixation 
point  in  the  middle  of  it.  Two  or  three  inches  to  the  right 
or  left  of  the  fixation  point  place  a  button  or  bit  of  black 
paper,  and,  a  foot  farther  and  nearer,  other  buttons  or  bits 
of  paper.  Then  leaning  over  the  table  so  as  to  bring  the 
eye  above  the  fixation  point,  try  to  place  the  three  buttons 
in  a  straight  line,  parallel  to  the  median  plane,  holding  the 
eye  steadily  upon  the  fixation  point.  Examination  of  the 
buttons  when  placed  will  show  that  the  middle  one  is  too 


near  the  fixation  mark  ;  i.e.,  the  attempt  to  make  a  straight 
line  has  resulted  in  a  curve  convex  toward  the  fixation  point. 
Try  also  with  the  buttons  in  lines  perpendicular  or  inclined 
to  the  median  plane. 

If  lines  convex  toward  the  fixation  point  appear  straight, 
lines  that  are  actually  straight  should  appear  concave.  On 
a  large  sheet  of  paper  draw  a  pair  of  parallel  lines  three  or 
four  inches  apart  and  two  or  three  feet  long.  Place  a  fixa- 
tion point  midway  of  their  length  and  half-way  between 
them  ;  fasten  the  paper  to  the  wall,  or  spread  it  on  the 


190      LABORATORY  COURSE  IN  PSYCHOLOGY.     fl72 

table,  and  observe  as  above.  Try  with  the  lines  vertical, 
horizontal,  and  in  oblique  positions.  In  a  spherical  field  of 
vision,  the  parallel  lines  of  this  experiment  would  be  repre- 
sented by  great  circles.  The  horizontal  pair,  for  example, 
would  have  their  poles  at  the  right  and  left  ends  of  the 
horizontal  axis  of  the  spherical  field,  and  their  planes  would 
make  equal  angles  above  and  below  the  plane  of  the  horizon.1 

It  is  obvious  that  changes  in  direction  which  make 
straight  lines  appear  curved  cannot  take  place  without  in- 
troducing slight  errors  of  distance  also.  The  shortest  dis- 
tances for  perception  are  the  curves  which  appear  straight. 

b.  Nature  of  Lines  that  Appear  Straight  in  Indirect  Vis- 
ion. It  would,  of  course,  be  possible  by  developing  the 
method  used  in  a  to  make  a  somewhat  exact  study  of  the 
nature  of  these  lines,  but  their  general  nature  may  be  found 
in  another  way.  In  the  hemispherical  field  of  regard  these 
lines  are  circles,  —  Helmholtz's  Circles  of  Direction.  The 
following  diagram  shows  the  projection  on  the  plane  field 
of  a  system  of  these  circles  of  direction.  For  use,  the  dia- 
gram must  be  enlarged  five  or  six  times.  It  should  be 
viewed  with  the  single  eye  opposite  its  centre,  and  at  a  dis- 
tance proportional  to  the  length  of  the  short  line  below  the 
diagram.*  In  order  to  fix  this  distance,  it  is  convenient  to 
cut  a  rod  of  such  length,  that  when  the  eye  is  at  the  right 
distance  the  rod  will  just  reach  from  the  outer  edge  of  the 
socket  of  the  eye  to  the  diagram.  When  the  head  is  brought 
into  the  proper  position,  and  the  eye  is  fixed  on  the  middle 
of  the  diagram,  the  lines  of  the  figure  will  appear  approxi- 
mately straight  and  parallel.  Try  with  the  diagram  in  the 
position  shown  below,  and  also  when  turned  so  as  to  make 


i  It  should  not  be  supposed  that  the  nai've  field  of  vision  is  hemispherical. 
The  field  is  neither  definitely  hemispherical  nor  definitely  anything  else,  except 
as  it  is  formed  hy  the  conditions  and  habits  of  vision.  It  is  here  spoken  of  as  a 
hemisphere  or  as  a  plane,  as  ease  in  exposition  may  require. 


172]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         191 

the  principal  lines  oblique.  Especial  care  should  be  taken 
to  avoid  movements  of  the  eyes,  for  a  new  interpretation 
of  the  curves  is  thus  introduced,  and  the  checker-board 
seems  concave  instead  of  plane.  This  disadvantage  may 
be  escaped  by  fixating  the  centre  of  the  diagram  till  a  sharp 
and  strong  after-image  is  secured,  and  then  observing  this 
with  closed  eyes. 


After  getting  the  general  effect,  the  observer  should  re- 
peat the  observation,  beginning  first  at  a  distance  greater 
than  that  just  used,  at  which  the  curvature  of  the  lines  can 


192       LABORATORY  COURSE  IN  PSYCHOLOGY.     [172 

easily  be  recognized,  and  then  slowly  approaching  till  a 
point  is  reached  where  the  lines  seem  straight  and  the 
squares  equal,  and  still  farther  till  the  curves  appear  to 
bend  the  other  way.  Test  the  distance  at  which  the  lines 
seem  straight  with  the  little  rod  mentioned  above ;  it  will 
generally  be  found  to  agree  approximately  with  the  dis- 
tance for  which  the  diagram  is  calculated.1 

It  thus  appears  that  the  projections  of  the  circles  of  di- 
rection are  the  lines  that  seem  straight  in  indirect  vision. 
These  circles  of  direction  are  lines  along  which  the  eye 
(when  moving  according  to  Listing's  Law)  can  carry  a  short 
after-image  without  causing  the  image  to  leave  the  line. 
They  are  in  this  particular,  for  the  eye  in  motion,  like 
straight  lines,  and  the  experiment  shows  that,  even  when 
the  eye  is  kept  still,  its  experiences  of  movement  exercise 
a  controlling  influence  on  its  perceptions.  (On  Listing's 
Law  and  the  circles  of  direction,  see  Ex.  131  b,  and  Ap- 
pendix I.) 

c.  Illusions  of  Form  in  Indirect  Vision.  Radial  dis- 
tances, as  might  be  inferred  from  the  diagram  of  5,  are 
more  decidedly  underestimated  than  distances  parallel  to 
the  margin  of  the  field.  Disks  of  cardboard  or  circles  when 
removed  a  little  from  the  fixation  point  appear  flattened. 
Try  with  a  six-inch  disk  held  at  arm's  length,  or  an  inch 
circle  on  cardboard  (or  the  larger  circle  in  the  last  figure 
in  Ex.  197  a).  Too  great  distance  from  the  fixation  point 
is  a  disadvantage ;  try  on  the  four  principal  meridians  of 
the  retina  at  distances  not  greater  than  the  diameter  of  the 
disk  or  circle. 


1  The  agreement  is  not  perfect,  and  there  are  perhaps,  in  addition,  individual 
differences  depending  on  the  exactness  with  which  the  eyes  follow  Listing's  Law. 
Helmholtz  finds  the  curvature  of  the  extreme  verticals  on  the  temporal  side  a 
little  too  great ;  and  Kiister,  working  by  a  slightly  different  method,  appears  to 
have  found  it  too  great  for  all  the  curves  (cited  by  Hering,  A,  370,  note). 


173]         VISUAL   PERCEPTION  OF  SPACE,  ETC.         193 

The  whole  field  of  vision  itself  appears  narrower  than  it 
really  is  ;  it  actually  covers  an  extent  of  about  180°,  and 
yet  under  favorable  circumstances,  as  when  looking  at  the 
dark  field  of  the  closed  eyes,  or  at  the  sky  in  the  absence 
of  all  landmarks,  the  extent  may  seem  not  much  over  90°. 

Helmholtz,  A,  G.  692  ff.,  Fr.  706  ff.  (551  ff.);  Wundt,  A,  4te 
Aufl.,  II.,  128  ff.;  Bering,  A,  369  ff.,  536  ff. 

173.  Directions  in  the  Monocular  Field  of  Begard.1  The 
observation  that  the  perceptions  of  the  eye  at  rest  are 
modified  by  those  of  the  eye  in  motion,  is  still  further  con- 
firmed by  the  similarity  of  other  phenomena  of  the  field  of 
regard  and  the  field  of  vision. 

a.  Straight  Lines  Viewed  with  the  Eyes  in  Secondary 
Positions.  Experiment  with  a  single  eye  and  a  long  ruler 
held  horizontally  before  an  even  wall  space  or  other  uni- 
form background.  Hold  the  flat  side  of  the  ruler  toward 
the  face,  and  about  a  foot  distant  from  it.  Try  first  with 
the  ruler  eight  or  ten  inches  above  the  primary  position 
of  the  line  of  sight  (cf.  p.  119),  running  the  eye  freely 
back  and  forth  along  the  edge,  and  observe  that  the  edge 
appears  curved  upward;  i.e.,  concave  below.  Try  with  the 
ruler  depressed  a  somewhat  greater  distance  below  the  pri- 
mary position,  and  observe  the  contrary  curvature.  Try 
also  with  the  ruler  vertical  and  '  to  the  right  and  left. 
Little  advantage  will  result  from  too  extreme  positions  of 
the  ruler.  The  curvature  to  be  observed  is  not  very  great ; 
but  that  it  is  due  to  the  visual  apparatus,  and  not  to  the 
ruler,  is  easy  to  show  by  turning  the  ruler  over,  which 
would  reverse  the  direction  of  an  actual  curvature  in  the 
ruler,  but  not  that  of  the  curvature  which  depends  on  the 


1  The  Field  of  Regard  is  the  extent  of  space  that  can  be  seen  directly  when 
the  eye  is  free  to  move ;  in  other  words,  the  field  within  which  the  fixation  point 
may  range. 


194      LABORATORY  COURSE  IN  PSYCHOLOGY.     [174 

eye.  Change  of  position  of  the  ruler  from  above  to  below 
the  primary  position  of  the  eye,  on  the  contrary,  reverses 
the  direction  of  the  curvature  due  to  the  eye,  but  not  a 
real  curvature  of  the  ruler.  Compare  the  results  here 
found  with  those  in  Ex.  172  a. 

The  occasion  of  the  illusion  is  the  rotation  of  the  eyes 
when  moved  from  point  to  point  in  secondary  positions. 
(Cf.  Ex.  131  b,  and  Appendix  I.)  When  the  eye  is  kept 
fixed  on  the  end  of  the  ruler,  or  moved  slowly,  the  ruler 
may  seem  slightly  tilted  instead  of  curved. 

Helmholtz,  A,  G.  686,  Fr.  699  (545);  Bering,  A,  536. 

174.  The  Retinal  Image  and  Perception  of  Size.  Accu- 
racy of  Discrimination.  The  perception  of  size  is  usually 
complicated  by  that  of  distance  as  well ;  but  when  objects 
are  at  the  same  distance,  their  relative  size  will  depend  on 
the  size  of  their  retinal  images,  if  the  eye  is  at  rest,  or  on 
that  and  the  extent  of  the  angles  through  which  the  eye 
must  be  moved  to  sweep  over  them,  if  it  is  in  motion.1 

a.  Accuracy  of  Comparison  with  the  Eyes  at  Rest. 
Test  with  Galton's  bar  and  the  krypteon  as  follows.  Place 
upon  the  middle  of  the  flap  of  the  instrument  a  small  point 
to  serve  as  a  fixation  point,  and  a  guide-mark  on  the  back- 
board to  help  in  placing  the  bar  so  that  its  division  thread 
may  be  each  time  exactly  behind  the  mark  on  the  flap. 

1  The  size  of  the  retinal  image  is  found,  as  explained  in  Ex.  117,  by  draw- 
ing lines  from  the  extreme  points  of  the  object  through  the  crossing  point  of 
the  lines  of  direction,  and  prolonging  them  to  the  retina.  The  angle  made  by 
these  lines  is  often  called  the  Visual  Angle.  This  construction,  however,  is 
exact  only  when  the  eye  is  exactly  accommodated.  When  the  eyes  are  not  ac- 
commodated, the  sighting  lines  should  be  used  to  form  the  angle  instead  of  the 
lines  of  direction.  And  when  objects  are  seen  by  sweeping  the  eye  over  them 
from  end  to  end,  the  lines  which  give  the  true  visual  angle  are  obviously  those 
from  the  extremities  of  the  object  to  the  centre  of  rotation  of  the  eye.  These 
various  kinds  of  visual  angles  differ  but  slightly  among  themselves,  and,  as  a 
matter  of  fact,  are  all  purely  artificial.  Immediate  perception  knows  nothing 
of  visual  angles  or  retinal  images,  but  only  things  seen. 


174]         VISUAL   PERCEPTION   OF  SPACE,    ETC.         195 

Adjust  the  Galton  bar  so  that  its  division  thread  is  in  the 
middle.  Place  it  in  the  krypteon,  and  cover  it  with  the 
flap.  Let  the  subject  fixate  the  point  on  the  flap;  and 
when  he  is  quite  ready,  let  him  quickly  turn  down  the  flap, 
and,  keeping  his  eyes  unmoved,  make  his  judgment  as  to 
the  equality  of  the  two  parts  of  the  bar.  If  the  parts 
seem  unequal,  a  constant  error  in  his  judgment  is  probable, 
and  the  setting  must  be  made  such  as  to  compensate  it. 
If  the  parts  seem  equal,  record  the  judgment,  remove  the 
bar,  and  alter  the  setting  slightly.  Replace  the  bar  as 
before,  with  the  division  thread  behind  the  fixation  mark, 
and  require  a  new  judgment.  Repeat  this  process,  gradu- 
ally increasing  the  displacement  until  the  subject  is  just 
able  to  recognize  a  difference  in  the  parts  of  the  bar. 
Record  the  difference  of  length  required  for  this  judgment, 
and  continue  the  experiment,  beginning  this  time,  however, 
with  the  parts  quite  distinctly  unequal  and  working  gradu- 
ally toward  equality. 

A  number  of  determinations  should  be  made  when  the 
thread  is  displaced  toward  the  right  and  toward  the  left, 
and  with  changes  toward  equality  and  away  from  it  —  an 
equal  number  of  each  kind  —  and  the  average  of  all  taken. 
The  ratio  of  the  just  observable  difference  to  the  length  of 
one  part  of  the  bar  is  the  measure  of  the  accuracy  of  dis- 
crimination required.  Averaging  the  results  separately  for 
the  cases  in  which  the  thread  is  displaced  towards  the 
right  and  towards  the  left,  will  show  the  constant  error  in 
judgment,  if  there  is  any.  It  might  seem  profitable  to 
furnish  the  subject  with  a  head-rest,  in  order  to  secure  a 
constant  distance  between  his  eyes  and  the  bar ;  but  there 
is  reason  to  think  this  relatively  unimportant  (v.  Kries, 
p.  187),  and  at  all  events,  it  is  not  necessary  for  casual 
testing.  Care  should  be  taken,  however,  that  the  distance 
is  not  such  as  to  bring  one  end  of  the  bar  into  the  part  of 


196       LABORATORY  COURSE  IN  PSYCHOLOGY.      [175 

the  field  corresponding  to  the  blind  spot.  Movements  of 
the  eyes  from  end  to  end  of  the  bar  must  be  excluded, 
and,  with  care  on  the  part  of  the  subject,  there  should  be 
no  great  difficulty  in  doing  so.  Of  course,  any  trials  in 
which  movements  occur  should  be  reported  and  dropped 
from  the  record.  If  more  perfect  exclusion  of  eye-move- 
ments is  desired,  it  may  be  obtained  by  placing  the  bar  in 
a  dark  box,  and  using  instantaneous  illumination. 

b.  Accuracy  of  Comparison  with  Movement  of  the  Eyes. 
Repeat  the  experiment  with  all  conditions  as  in  a,  except 
that  after  the  showing  of  the  bar  the  subject  be  allowed  to 
move  his  eyes  freely  in  comparing  the  parts.  Compare 
the  results  found  in  a  and  b. 

Wundt,  A,  4te  Aufl.,  II.,  132  ff.;  Helmlioltz,  A,  G.  682  ff.,  Fr.  695 
ff.  (541  ff.);  Miinsterberg;  and  the  literature  cited  by  them.  For 
measurements  of  a  similar  kind  upon  squares,  see  Warren  and  Shaw 
(240) ;  for  measurements  on  circles,  and  for  effect  of  color  on  size, 
see  Quantz.  For  experiments  on  the  exactness  with  which  extents 
can  be  compared  when  their  distances  from  the  eye,  and  so  their 
retinal  images,  are  unequal,  see  Fechner,  II.,  311  f. ;  Martius;  and 
v.  Kries,  187  ff. 

175.  The  Retinal  Image  and  the  Perception  of  Size  : 
Ordinary  Seeing.  In  the  absence  of  other  determining  cir- 
cumstances, large  retinal  images  are  taken  to  belong  to 
large  objects,  and  small  to  small.  Undetermined  cases  are, 
however,  extremely  rare. 

a.  Known  Objects  are  Generally  Perceived  as  of  a  Con- 
stant Size,  Irrespective  of  the  Size  of  Their  Retinal  Images. 
Hold  the  hand  eight  inches  from  the  face,  and  notice  its 
size ;  then  move  it  to  sixteen  inches,  and  observe  that  its 
apparent  size  remains  the  same,  despite  the  fact  that  its 
retinal  image  has  now  only  one-half  its  former  length  and 
only  one-quarter  its  area.  On  further  removal  to  twenty- 
four  inches,  the  apparent  size  is  still  the  same.  This  con- 


175]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         197 

stancy  is  found  in  estimating  the  height  of  men,  domestic 
animals,  and  familiar  objects  generally,  and  is  frequently 
made  use  of  by  painters,  who  introduce  the  figures  of  men 
and  other  well-known  objects  to  suggest  indirectly  the  size 
of  objects  near  which  they  are  placed.1  See  also  Ex.  124  d, 
where  a  change  in  the  size  of  the  retinal  image  causes  a 
change  in  the  color,  but  not  in  the  apparent  size  of  the 
object. 

b.  When  the  objects  are  equally  familiar,  an  important 
part  is  played  by  attention  in  determining  which  shall  be 
taken  as  the  measure  of  the  other.     This  is  easily  shown 
with  two  fingers,  one  held  at  eight,  the  other  at  twenty-four 
inches.     Steady  looking  at  the  farther  finger  makes  the 
nearer  look  larger  than  normal ;  and  looking  at  the  nearer, 
makes  the  farther  look  smaller. 

c.  Another  experiment  which  shows  the  same  indepen- 
dence of  the  retinal  image  is  cited  by  Helm  hoi  tz   from 
Smith's  Opticks.     Place  in  the  focus  of  a  convex  lens  a 
wafer,  a  printed  letter,  or  any  other  small  object,  and  view 
it  at  different  distances  from  the  lens.     As  the  distance  in- 
creases, the  object  will  seem  to  enlarge  until  it  fills  the  lens 
completely.     The  fact  is,  however,  that  its  image  remains 
approximately  constant  in  size  (since  the  rays  from  it  are 
made  parallel  by  the  lens),  while  the  image  of  the  lens 
itself,  and  of  all  other  objects  in  the  visual  field,  decreases 
in  size. 

Hering,  B,  14  f  ;  Rivers  ;  Helmholtz,  A,  G.  839,  Fr.  871  (689). 


1  In  somewhat  the  same  way  a  spire  or  tree  may  serve  as  a  measure  for  the 
disk  of  the  sun  or  moon  rising  or  setting  behind  it,  with  the  result  that  the  latter 
seems  larger  than  when  such  comparison  is  impossible.  This,  however,  is  by  no 
means  the  only  element  in  the  illusion.  The  flattened  form  of  the  sky  —  its'elf 
the  resultant  of  several  causes  —  also  co-operates  by  making  the  sun  or  moon  at 
the  horizon  seem  farther  away,  and  therefore  larger.  The  matter  may  be  fol- 
lowed in  Helmholtz  ;  Aubert ;  Wundt ;  Filehne  ;  and  in  a  discussion  by  Lecha- 
las  and  others  in  the  Revue  philosopliique,  juillet,  1888-fevrier,  1889. 


198      LABORATORY  COURSE  IN  PSYCHOLOGY.      [176 

176.  The  Ketinal  Image  and  Perceptions  of  Size  and 
Distance.  A  circumstance  that  very  frequently  determines 
the  apparent  size  of  an  object  is  its  apparent  distance  ;  or, 
more  generally,  size  and  distance  are  mutually  .determining. 
If  the  apparent  distance  is  constant,  the  apparent  size  of 
the  object  changes  directly  with  the  size  of  the  retinal  im- 
age ;  while  if  the  apparent  size  is  constant,  the  apparent 
distance  changes  inversely  with  the  image.  These  are  facts 
of  very  common  observation.  In  the  laboratory  they  may 
be  demonstrated  as  follows  :  — 

Look  at  a  portion  of  a  page  of  print  through  an  ordinary 
magnifying-glass,  holding  the  glass  near  the  page,  so  that  a 
good  deal  of  the  latter  can  be  seen  outside  the  lens.  The 
retinal  image  of  the  part  seen  through  the  lens  is  enlarged, 
but  the  parts  of  the  page  seen  outside  the  lens  fix  the 
distance  for  the  whole,  so  that  the  letters  seem  enlarged. 
On  the  contrary,  when  an  opera-glass  or  a  telescope  is  used 
for  a  distant  object,  the  eye  is  brought  so  close  to  the  eye- 
piece that  nearly  all  the  visual  field,  except  that  seen 
through  the  instrument,  is  cut  off.  The  result  is,  then, 
that  objects  appear  nearer,  and  but  little,  if  any,  larger. 
The  effect  is  equally  clear  when  the  retinal  images  are  re- 
duced by  using  a  double  concave  lens  in  the  first  case,  and 
by  looking  through  the  opera-glasses  from  the  big  end  in 
the  second. 

Hillebrand,  B,  121  f. 

Perception  of  the  Position  and  Movement  of  the  Lines 
of  Regard.  The  importance  of  eye-movements  in  space 
perception  is  clear  from  previous  experiments,  and  will 
be  still  further  emphasized  by  several  of  the  Geometrical 
Illusions  below ;  but  the  manner  in  which  they  play  their 
part  is  anything  but  clear.  It  has  generally  been  assumed 
that  they  give  rise  to  kiiuesthetic  sensations  of  some  kind, 


177]  VISUAL  PERCEPTION  OF  SPACE,   ETC.        199 

and  that  through  these  the  changing  positions  of  the  eyes 
are  perceived.  This,  however,  is  doubtful,  for  direct  ex- 
periment shows  that  perception  of  the  positions  of  the 
eyes  when  retinal  sensations  are  excluded  is  very  defective. 
(Cf.  Hering,  B,  30  ff.)  This  has  already  been  noticed  in 
the  case  of  eye-movements  from  dizziness  (Ex.  50),  and 
other  cases  are  given  below.  What  the  true  explanation 
is  —  whether  eye-movements  are  effective  solely  by  the 
changes  they  cause  in  the  retinal  impressions,  or  by  some 
more  direct  means  —  is  something  yet  to  be  settled. 

Perception  of  the  position  of  the  eyes  or  of  the  lines  of 
regard  may  be  absolute  or  relative.  It  is  obvious  that  per- 
ception of  the  absolute  position  can  only  mean  the  co-ordi- 
nation of  that  perception  with  those  of  some  other  sense  or 
senses,  and  the  term  is  used  here  with  that  meaning  only. 
Perception  of  the  relative  position  will  mean  co-ordination 
with  other  perceptions  of  the  same  sense.  Relative  direc- 
tion in  the  field  of  regard,  for  example,  is  measured  from 
the  primary  position  of  the  lines  of  regard,  which  is  prac- 
tically that  taken  by  those  lines  when  the  head  is  erect 
and  the  eyes  are  fixed  on  the  horizon. 

177.  Normal  and  Forced  Movements  of  the  Eyes.  When 
the  line  of  sight  is  shifted  voluntarily,  objects  seen  appear 
at  rest,  and  the  eye,  so  far  as  it  is  regarded,  in  motion. 
When,  however,  the  shifting  is  involuntary,  as  when  the 
eyes  are  forced  to  move  by  pressure  of  the  fingers,  or  by 
inner  causes,  as  in  dizziness  or  dropping  to  sleep,  objects 
seem  to  move.  Close  one  eye,  and  take  between  the  thumb 
and  finger  a  fold  of  the  skin  at  the  outer  edge  of  the  or- 
bit of  the  other  eye,  and  draw  it  gently  outward.  The 
eye  itself  is  thus  drawn  slightly  in  the  same  direction. 
Objects  in  the  field  seem  to  move  a  little  in  the  opposite 
direction.  Get  a  strong  after-image  of  the  window  in  one 


200       LABORATORY  COURSE  IN  PSYCHOLOGY.      [179 

eye,  close  and  cover  the  same  eye,  and  repeat  the  traction 
of  the  skin  on  that  side.  The  after-image  will  not  appear 
to  move.1 

Such  a  result  appears  to  exclude  sensations  from  change 
in  the  eye  muscles,  and,  indeed,  from  any  other  external 
change  in  the  eye  from  participation  in  any  of  the  finer 
spatial  perceptions. 

Helmholtz,  A,  G.  743,  Fr.  763  f.  (600). 

178.  Fixation  in  Complete  Darkness.     It  is  difficult,  if 
not  impossible,  to  hold  the  eyes  in  a  given  position  without 
the  assistance  of  the  retinal  sensations.     Arrange  the  dark 
box  for  binocular  vision,  and  insert  at  the  back  a  slide  with 
a  single  hole.     Fasten  over  the  hole  a  bit  of  black  card- 
board, so  as  to  exclude  all  light.     Make  a  small  pin-hole  in 
the  card,  which  will  appear,  when  seen  from  the  front,  as 
a  minute  point  of  light.     Provide,  also,  another  piece  of 
black  cardboard,  which  can  be  used  from  time  to  time  to 
cover  the  hole  and  cut  off  its  light.     Bring  the  eyes  into 
position,  and  fixate  the  pin-hole  for  a  second  or  two ;  then 
cut  off  its  light,  and  try  to  maintain  the  fixation.     After 
ten  or  twenty  seconds  allow  it  to  appear  again,  and  notice 
whether  it  comes  at  the  point  expected.     Generally  it  does 
not.     If  the  kinaesthetic  sensations  of  the  eyes  were  acute, 
such  errors  ought  not  to  occur.     For  other  instances  of  un- 
perceived  movements  of  the  eyes,  see  the  experiments  on 
illusions  of  movement,  below. 

Hillebrand,  B,  150 ;  Helmholtz,  A,  G.  757  f.,  Fr.  779  f.  (613). 

179.  False  Location  of  After-images. 

a.  If  one  fixates  for  a  few  seconds  a  small  gas-flame  or 
other  bright  object  in  a  darkened  room,  and  then,  keeping 

1  It  is  probable  that  moving  of  the  eye  through  a  great  enough  angle  in  this 
way  would  cause  some  apparent  movement  of  the  after-image ;  such,  at  least, 
is  the  case  when  the  eyes  are  moved  involuntarily  in  dizziness. 


179]          VISUAL  PERCEPTION  OF  SPACE,   ETC.        201 

his  head  unmoved,  looks  quickly  away,  he  will  observe  a 
long  positive  after-image  streak  connecting  the  flame  with 
the  new  fixation  point.  This  has  already  been  used  in  Ex. 
132  as  a  means  of  studying  the  movements  of  the  eyes.  It 
sometimes  happens,  however  —  especially  when  the  head 
is  moved  with  the  eyes,  and  the  movement  is  sudden  —  that 
the  after-image  is  not  correctly  located,  but  referred  to  the 
side  of  the  flame  opposite  to  that  on  which  the  new  fixation 
point  is  found.  It  often  seems  to  shoot  out,  as  it  were, 
from  the  flame.  If  the  movement  has  been  upward,  the 
after-image  seems  to  lie  below  the  flame ;  if  downward, 
above  the  flame,  and  similarly  with  other  directions.  As 
Mach  expresses  it,  the  image  appears  with  the  place-marks 
that  belong  to  the  old  and  not  to  the  new  position  of  the 
eyes.  Lesser  degrees  of  false  location,  in  which  the  after- 
image streak  is  partly  on  the  same  side  of  the  flame  as  the 
movement,  and  partly  on  the  other,  are  also  observed ;  and 
somewhat  similar  effects  are  to  be  seen  when  the  move- 
ment is  toward  the  flame  from  some  other  fixation  point. 

The  explanations  offered  for  the  phenomenon  do  not 
seem  entirely  satisfactory,  but  the  essential  factor  is  prob- 
ably defective  perception  of  the  actual  movements  of  the 
eyes. 

b.  Lagging  of  the  Eye  when  the  Head  is  Turned.  Fix- 
ate  a  flame  or  other  bright  object  for  a  single  second  or 
less ;  then  close  the  eyes,  and  quickly  turn  the  head  30°  or 
40°  to  the  right  or  left,  or  up  or  down.  The  after-image 
(often  positive)  will  appear  in  the  original  direction  of  the 
object.  Repeat  the  fixation,  continuing  it  this  time  for 
twenty  seconds.  The  after-image  (negative)  will  appear  to 
turn  with  the  head.  Intermediate  positions  also  will  some- 
times be  noticed.  Miinsterberg  and  Campbell  report  that 
many  persons  are  unsuccessful  in  getting  this  result. 

On  a,  Mach,  A\  Lipps,  A.     On  6,  Miinsterberg  and  Campbell. 

Or  Tt.b      * 


202      LABORATORY  COURSE  IN  PSYCHOLOGY.      [180 

180.    Locations  in  the  Indirect  Field  of  Regard. 

a.  Use  the  campimeter,  adjusting   the  head-rest  about 
20  cm.  from  the  vertical  plane.     Make  a  distinct  fixation 
mark  before  the  eye,  and  another  at  a  distance  of  15  to  18 
cm.  to  the  right  or  left.     Let  the  subject  bring  both  his 
hands  into  symmetrical  positions  near  the  median  plane  — 
e.g.,  on  either  side  of  the  foot  of  the  head-rest  —  and  close 
his  eyes.     At  command,  let  him  open  his  eyes,  take  a  care- 
ful observation  of  the  distance  of  the  side  fixation  mark 
from  the  median  one,  close  his  eyes  again,  and  try  to  touch 
the  side  mark  by  a  rather  quick  movement  of  the  hand 
upon  the  same  side.     Note  the  extent  and  direction  of  the 
error,  and  repeat  the  experiment,  being  careful  always  that 
the  subject  does  not  shift  his  head,  and  that  he  keeps  his 
eyes   closed,   except  when  judging  the  separation   of  the 
marks ;    this    last,  in  order  that  he  may  remain  in  igno- 
rance of  the  extent  of  his  error.     Try  several  times  on 
either  side.     The  subject  will  generally  overestimate. 

b.  Kepeat  the  experiment,  this  time  turning  the  head 
with  the  eyes,  instead  of  the  eyes  alone,  and  keeping  the 
head  turned  during  the  touching,  the  eyes  of  course  being 
closed  as  before.     Also,  with  the  head  straight,  try  touch- 
ing the  median  fixation  mark.     In  both  cases  the  error  will 
be  small  or  absent,  showing  that  the  defect  is  visual  (eye- 
muscles),  and  not  in  the  arm.     Loeb  finds  a  similar  error 
in  touching  points  in  the  peripheral  field  of  vision ;  i.e., 
points  seen  indirectly  without  eye-movements. 

Loeb's  explanation  is  that  the  eye-muscles  are  less  and 
less  responsive,  and  require  a  greater  and  greater  innerva- 
tional  discharge  for  a  given  response  as  they  become  more 
and  more  contracted.  The  position  of  the  eyes  (and  the 
location  of  the  fixation  mark)  is  judged  by  the  "  volitional 
impulse "  required,  and  therefore  overestimated.  For  a 
similar  tendency  to  overestimate  the  position  of  the  eye, 
see  Hering,  A,  444.  Of.  also  Ex.  36. 


182]         VISUAL  PERCEPTION  OF  SPACE,    ETC.         203 

On  a  and  &,  Loeb,  B,  21  ff.  On  6,  Bowditch  and  Southard  (quan- 
titative results  for  touching  under  various  conditions)  ;  Exner,  322. 

181.  Co-ordination  of  Vision  and  Touch.     In  the  ordi- 
nary use  of  the  eyes  the  visual  and  tactual  locations  of 
objects  coincide  very  well  for  direct  vision.     It  is  easy, 
however,  to  produce  a  dislocation  of  one  with  reference  to 
the  other,  and  eventually  a  new  adjustment. 

Lay  on  the  table,  at  a  convenient  distance,  a  button  or 
other  small  object.  Observe  the  button  for  a  second  or 
two  through  a  prism  of  10-20°  angle,  then  close  the  eye 
and  attempt  to  touch  the  button  by  a  rapid  movement  of 
the  hand.  The  hand  will  be  found  to  have  erred  on  the 
side  toward  which  the  field  has  been  shifted  by  the  prism. 
A  few  trials  with  the  eye  open  will  enable  the  observer 
to  touch  the  button  with  certainty.  Continue  the  practice, 
however,  for  a  few  moments.  Then  remove  the  prism, 
observe  the  button  with  free  eyes  ;  close  them,  and  try 
again  to  touch  the  button  by  a  rapid  hand-movement.  An 
error  will  again  be  found,  but  in  the  opposite  direction, 
showing  that  a  new  co-ordination  of  visual  and  touch  space 
has  been  formed. 

Helmholtz,  A,  G.  745 ;  Fr.  765  f.  (601  f.). 

182.  Perception  of  Depth  by  Means  of  Accommodation. 
Whether  the  direct  muscular  effort  of  accommodation  has 
any  effect,  apart  from  changes  of  the  retinal  image  or  asso- 
ciated tendencies  to  binocular  convergence  of  the  lines  of 
sight,  has  been  questioned.     The  whole  "problem,  both  as 
to  judgments  depending  on  normal  accommodation  and  on 
that  required  by  chromatic  aberration,  is  still  sub  judice, 
and  will  not  be  followed  further  here.1 

1  Experiments  have  been  made  on  the  matter  by  Wundt  (A,  4te  Aufl.,  IT., 
107  ;  B,  105  ff.),  by  Hillebrand,  Dixon,  Rouse,  Arrer,  and  Bourdon.  For  influence 
of  chromatic  aberration  see  Thompson,  A  ;  but  his  results  are  hard  to  verify. 


204       LABORATORY   COURSE  IN  PSYCHOLOGY.     [183 

That  changes  in  accommodation  cause  changes  in  appar- 
ent size  and  distance  has,  however,  long  been  known.  If, 
while  attention  is  given  to  a  distant  object,  e.  g.,  a  house 
or  tree,  the  eye  is  quickly  accommodated  for  a  near  point, 
the  distant  object  will  appear  to  withdraw  and  diminish  in 
size.  If  the  operator  is  not  able  to  accommodate  volunta- 
rily, the  experiment  may  easily  be  made  by  letting  him 
stand  close  to  the  window  and  select  a  spot  on  the  glass 
for  a  point  of  near  fixation.  Aubert  finds  a  difference  in 
result  with  objects  of  unknown  size.  These  are  reduced 
in  size,  and  given  an  extremely  near  location.  Accommo- 
dation for  a  near  point,  while  looking  through  a  pin-hole  in 
a  card  held  close  before  the  eye,  shows  the  same  result 
somewhat  more  easily,  but  heightened  perhaps  by  other 
conditions.  Carrying  the  card  toward  the  object  produces 
still  further  diminution  in  size.  Aubert  finds  a  change  of 
apparent  distance  due  to  the  efforts  of  accommodation 
when  no  actual  change  in  accommodation  results. 

Helmholtz,  A,  G.  119;  Fr.  127  (97);  Aubert,  A,  601-602,  627; 
Stevens,  .B,  346  f . ;  Kirschmann,  452  ;  Eivers. 

183.  Perception  of  Depth  by  Means  of  Intervening  Ob- 
jects. 

a.  Several  of  the  monocular  criteria  of  distance  are  better 
observed  in  the  casual  use  of  the  eyes  than  in  specific  ex- 
periments, and  this  among  the  rest.  The  following  figures, 
however,  show  something  of  the  tendency.  We  are  more 
inclined  to  regard  the  rings  in  A  as  complete  and  interlaced 
than  as  broken  and  carefully  laid  together.  In  the  second 
figure  the  effect  is  still  stronger,  because  it  is  still  more 
difficult  to  conceive  the  arch  in  the  same  plane  with  the 
column  and  fitting  exactly  into  its  irregular  outline. 

The  multitude  of  objects  intervening  between  the  eye 
and  the  horizon,  together  with  their  known  size  and  dis- 


183]        VISUAL  PERCEPTION  OF  SPACE,   ETC. 


205 


tance,  doubtless  contributes  also  to  the  flattened  appear- 
ance of  the  dome  of  the  sky. 

b.  Most  people  have  great  difficulty  in  seeing  the  image 
produced  by  a  concave  mirror  in  front  of  the  mirror-sur- 
face, though  in  most  cases  it  is  actually  so  located.  The 
apparent  interference  of  intervening  objects  combines  with 
the  customary  location  of  mirror-images  behind  the  mirror- 
surface  to  produce  the  false  location.  The  experiment  may 
be  made  as  follows :  At  a  distance  in  front  of  a  concave 
mirror,  somewhat  less  than  double  its  focal  distance,  is  set 


up  a  figure  like  that  below,  cut  from  cardboard  and  black- 
ened on  both  sides,  or  even  an  ordinary  retort  ring  of  small 
size.  The  observer  takes  his  position  still  further  from  the 
mirror  in  the  line  passing  through  its  centre  and  the  centre 
of  the  ring,  and,  if  the  adjustments  are  correct,  sees  float- 
ing in  the  air,  a  few  inches  in  front  of  the  actual  figure,  an 
enlarged  and  inverted  image  of  it,  so  long,  at  least,  as  he 
observes  with  both  eyes.  The  instant,  however,  that  he 
looks  with  a  single  eye,  the  image  drops  back  to  the  mirror 
surface  or  beyond.  The  rays  of  the  figure,  the  spots  on 
the  mirror,  which  are  plainly  seen  through  the  floating 


206       LABORATORY  COURSE  IN  PSYCHOLOGY.     [184 

image,  and  the  frame  of  the  mirror,  which  cuts  the  image 
off  at  the  sides,  all  conspire  to  make  the  image  seem  behind 
instead  of  in  front.  If  the  observer 
has  difficulty  in  getting  the  binocular 
location,  a  little  swaying  of  the  head 
from  side  to  side,  which  causes  the 
image  to  shift  with  reference  to  the 
mirror  and  the  figure,  may  be  help- 
ful. 

A  similar  experiment  may  be  made 
with  a  suitably  adjusted  convex  lens. 

Helmholtz,  A,  G.  768  f. ;  Fr.  793  (624  f . ) ; 
Sully,  80  f . 

184.    Perception  of  "Relief  by  Means  of  Shadows. 

a.  The  effect  of  shadows  is  finely  shown  by  a  mask  col- 
ored alike  within  and  without.  Place  the  mask,  with  the 
hollow  side  toward  the  observer,  in  such  a  position  that  the 
light  falls  full  upon  it  and  no  shadows  are  cast  inside  it. 
Let  the  observer  regard  it  with  a  single  eye  from  a  distance 
of  six  or  eight  yards.  He  will  find  it  difficult,  or  even 
impossible,  to  see  the  mask  in  its  true  concave  condition, 
preponderant  experience  apparently  dictating  the  opposite 
result  in  perception.  If,  however,  the  position  of  the  mask 
is  so  changed  that  the  light  falls  into  it  obliquely,  the 
shadows  immediately  betray  the  concavity,  and  no  difficulty 
is  found,  except,  perhaps,  with  the  nose,  which  lies  wholly 
in  the  shadow. 

Medallions  with  heads  in  low  relief,  when  lighted  equally 
from  all  sides,  can  with  a  little  effort  be  seen  either  convex 
or  concave,  — cameo  or  intaglio.  The  presence  of  unequal 
illumination  and  cross  shadows  makes  this  more  difficult. 
A  sheet  of  paper  folded  like  a  half-open  book,  and  set  up 
vertically,  shows  somewhat  the  same  effect,  especially  if 


184]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         207 

the  lower  end  is  covered  so  that  its  contact  with  the  table 
cannot  be  seen.  Cf.  Fig.  N,  Ex.  202  and  Ex.  203. 

b.  Waller's  Experiment.  In  the  following  experiment 
dark  borders  resembling  shadows  lead  to  an  illusion  of  ele- 
vation or  depression.  Cut  a  piece  of  cardboard  eight  inches 
long  by  four  wide  ;  cover  half  of  it  smoothly  with  red 
paper  and  half  with  blue.  On  the  red  paper  paste  several 
strips  of  blue,  and  on  the  blue  several  strips  of  red,  strips 
a  quarter  of  an  inch  wide  by  two  long;  or,  better,  put 
on  concentric  rings  of  the  specified  colors,  leaving  spaces 
between  at  least  equal  to  the  breadth  of  the  rings.  The 
gummed  rings  used  by  kindergartners  serve  excellently 
for  the  purpose.  Place  the  diagram  thus  made  in  such  a 
position  that  it  shall  be  strongly  illuminated  from  the  right 
side,  and  view  it  from  a  distance  of  two  or  three  yards 
with  a  single  eye,  covering  half  the  pupil  with  a  bit  of 
black  cardboard,  or,  better,  through  a  hole  in  the  cardboard 
about  two  mm.  square,  the  card  being  shifted  toward  the 
nose  or  the  temple  to  imitate  a  similar  dislocation  of  the 
pupil. 

If  the  temporal  half  of  the  right  pupil  is  covered,  the  red 
rings  will  appear  to  stand  out  slightly  from  their  ground ; 
the  blue  will  appear  to  lie  somewhat  depressed  in  theirs. 
If  the  nasal  half  of  the  pupil  is  covered,  the  red  will  be 
depressed  and  the  blue  elevated.  The  same  is  true  for  the 
left  eye  if  the  terms  nasal  and  temporal  are  interchanged. 
Notice  in  each  case  the  apparent  distribution  of  light  and 
shade.  Changing  the  direction  of  illumination  sometimes 
reverses  the  phenomenon.  The  experiment  may  be  some- 
what easier  when  the  observer  looks  through  a  piece  of  blue 
glass  (or  violet  or  purple  gelatine)  held  close  before  the 
eye.  The  purpose  of  the  blue  glass  is  simply  to  make  the 
blue  and  red  of  the  papers  used  in  the  diagram  purer. 
The  edge  of  the  card  that  covers  the  pupil  may  be  black- 


208       LABORATORY  COURSE  IN  PSYCHOLOGY.     [184 

eried  with  advantage,  and  slight  movements  of  the  card 
may  also  prove  helpful. 

The  illusion  depends  upon  the  interpretation  of  the  appar- 
ent shadows  and  high  lights.  These  arise  from  chromatic 
aberration,  which  is  made  much  more  apparent  than  in 
the  normal  eye  by  half -covering  the  pupil.  The  matter  will 
be  made  clear  by  an  examination  of  the  figures  opposite. 
In  discussing  the  figures,  it  is  assumed  that  the  colors  in 
question  are  perfectly  pure,  and  that  the  right  eye  is  taken 
for  experiment,  with  the  temporal  half  of  the  pupil  covered. 

It  is  impossible  to  accommodate  the  eye  at  the  same 
time  for  both  red  and  blue ;  if  the  red  rays  are  brought  to 
a  focus  on  the  retina,  the  blue  rays  are  focused  in  front  of 
it ;  if  the  blue  rays  are  brought  to  a  focus  on  the  retina,  the 
red  rays  are  focused  behind  it.  In  the  figures  opposite,  L 
represents  the  line  of  demarcation  between  a  red  area  and  a 
blue  area ;  in  Fig.  A  the  eye  is  accommodated  for  the  red ; 
in  B  for  the  blue.  In  A  the  edge  of  the  red  in  the  retinal 
image  lies  at  I,  the  edge  of  the  blue  at  a,  which,  when 
referred  outward  on  the  line  of  direction  a  L',  locates  the 
blue  edge  at  Lr,  a  shifting  toward  the  left,  which  causes  an 
over-lapping  of  the  red  and  blue,  since  the  red  edge  is  per- 
ceived at  L  in  its  true  position.  Similarly  in  Fig.  B  ac- 
commodation for  the  blue  causes  an  apparent  shifting  of 
the  location  of  the  edge  of  the  red  to  L",  a  shifting  toward 
the  right.  Any  intermediate  degree  of  accommodation 
would  cause  a  shifting  of  both  the  red  and  the  blue  in 
opposite  directions.  In  Fig.  C  is  also  shown  another  result 
of  such  shiftings.  Assume  that  abed  represents  a  red 
strip  on  a  blue  ground.  When  this  combination  is  viewed 
with  the  temporal  half  of  the  right  pupil  covered,  there 
is  a  mutual  shifting  of  the  colors,  and  the  strip  abed 
appears  in  the  position  efg  h.  The  result  is  a  summation 
of  the  colors  in  the  region  bfg  c,  and  an  absence  of  all 


184]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         209 

color  (darkness)  in  the  area  a  e  h  d.  The  region  of  summa- 
tion is  taken  as  a  high  light,  the  region  of  darkness  as  a 
shadow  —  a  condition  of  things  that  would  be  exactly  par- 
alleled if  a  slight  elevation  existed  in  a  field  illuminated 
obliquely  from  the  right.1  In  a  way  entirely  similar  to 


L'  L 


L  L' 


b  f 


B 


that  just  used,  the  cases  of  blue  figures  on  a  red  ground, 
of  vision  with  the  nasal  half  of  the  pupil  covered,  and  of 
vision  with  the  left  eye,  can  readily  be  explained. 

On  a,  Helmholtz,  A,  G.  772;  Fr.  797  f  (628);  Bowditch;  Oppel, 
B.     On  &,  Einthoven.    Brewster. 

1  Einthoven,  who  has  investigated  this  phenomenon  carefully  with  a  nearly 
identical  setting  of  the  experiment,  considers  the  direction  of  illumination  of 
little  consequence.  In  the  setting  here  used,  it  has  seemed  to  me  rather  impor- 
tant, though  not  absolutely  determining.  The  illusion  is  one  of  perceptive  in- 
terpretation, where  individual  differences  are  to  he  expected.  The  main  point 
remains,  however,  in  either  case,  that  illusory  shadows  cause  a  plastic  inter- 
pretation of  the  figure. 


210       LABORATORY  COURSE  IN  PSYCHOLOGY.     [185 

185.  Perception  of  Relief  as  Influenced  by  Other  Per- 
ceptions and  Ideas.    Lay  on  the  table  a  short  strip  of  paper 
torn  roughly  across  one  end.     Look  with  a  single  eye  at  the 
paper  through  a  convex  lens  of  short  focus,  from  a  distance 
of  two  or  three  feet,  in  such  a  way  that  the  line  of  sight 
makes  an  angle  of  about  45°  with  the  surface  of  the  table. 
The  lens  will  present  an  inverted  image  of  the  strip.    This 
image,  if  exactly  perceived,  ought   to  appear  vertical,  or 
nearly  so ;  it  will  seem,  however,  nearly  horizontal,  and  the 
end  which  is  actually  farther  away  will  seem  the  nearer. 
It  will  be  found  advantageous  to  have  the  image  of  the  paper 
nearly  fill  the  lens.     Repeat  the  experiment,  this  time  stick- 
ing a  pin  vertically  into  the  paper.     This  will  favor  a  truer 
perception  of  the  position  of  the  image.    The  false  location  of 
the  paper  depends  on  the  pre-conception  that  the  table  sur- 
face, as  seen  through  the  lens,  still  belongs  to  the  rest  of  the 
surface  as  seen  outside  of  the  lens  and  known  by  sight  and 
touch.    To  harmonize  with  this  the  paper  must  seem  turned 
end  for  end.     With  the  aid  of  the  pin,  it  takes  a  vertical 
position,  and  the  end  which  is  actually  nearer  seems  so  still. 

When  figures  in  shallow  relief  are  thus  viewed,  they  will 
also  seem  horizontal,  if  little  or  nothing  of  their  surround- 
ings is  seen  at  the  same  time,  and  often  show  changes 
from  depression  to  elevation  or  vice  versa.  If  they  assume 
the  vertical  position  their  relief  is  seen  correctly.  The 
changes  depend  somewhat  on  the  object  represented,  me- 
dallions easily  changing  from  intaglio  to  cameo  form,  but 
not  so  readily  from  cameo  to  intaglio.  Shadows  are  very 
important  here,  and  their  effect  can  be  finely  shown  when 
such  figures  are  tried  by  artificial  light.  Cf.  Ex.  203. 

Helmholtz,  A,  G.  773;  Fr.  798  (628);  Brewster,  B. 

186.  The  Perception  of  Form  and  Distance  in  Inverted 
Pictures  and  with  Inverted  Head. 


186]        VISUAL  PERCEPTION  OF  SPACE,    ETC.         211 

a.  Form  and  Distance  in  Inverted  Pictures.     Pictures 
produce  a  decidedly  different  effect  when  examined  upside 
down.     Try  with  any  convenient  set  of  pictures,  including 
both  portraits  and  landscapes.     The  effect  of  inversion  is 
strikingly  shown  in  the  "  topsy-turvy  "  picture  books  com- 
mon a   year  or  two  ago,  and  is  true  to  a  certain  extent 
even   of  familiar    portraits.     In    inverted   landscapes,  be- 
sides the  general  strangeness  and  a  strong  tendency  to  take 
sky  for  water  and  vice  versa,  there  is  often  an  imperfect 
perception  of  distance  which  results,  when  the  picture  is 
righted,   in  an  apparent  retreat  of   objects  in  the  back- 
ground.     This  is  due,  in  the  opinion  of  Filehne,  to  our 
vastly  preponderant  experience  in  the  recognition  of  per- 
spective effects  on  the  ground  and  below  the  horizon  line.1 
In  the   inverted  picture,  lines  that  usually  converge  up- 
ward converge  downward,   and  only  suggest  imperfectly 
their  usual  interpretation.     Dr.  Margaret  Washburn  finds 
many  cases  of  a  reverse  effect,  the  background  seeming  at 
a  greater  distance  in  the  inverted  position,  and  suggests  in 
explanation  a  different  estimate  of  size  in  the  upper  and 
lower  parts  of  the  field,  similar  to  that  in  the  case  of  the 
geometrical  illusions  of  Ex.  196,  below. 

b.  Distance  with  Inverted  Head.    When  the  actual  land- 
scape is  viewed  with  inverted  head,  by  looking  under  the 
arm  or  between  the  legs,  the  appearance  of  things  is  con- 
siderably changed ;   the   movements   of  men  and  animals 
seem  unfamiliar,  colors  are  often  brightened,  and  illusions 
\vith  regard  to  distance  are  experienced. 

Try  the  experiment,  taking  pains  to  have  the  eyes  at  the 
same  height  above  the  ground  in  both  the  inverted  and 
erect  positions.  If  this  last  is  not  regarded,  an  increase 


1  Filehne  also  uses  the  same  principle  for  explaining  the  flattened  shape  of 
the  sky  and  the  enlargement  of  the  moon  at  the  horizon. 


212       LABORATORY  COURSE  IN  PSYCHOLOGY.     [186 

of  distance  in  the  inverted  position  and  a  decrease  in  the 
erect  will  be  observed  which  is  not  directly  due  to  the  in- 
version of  the  head. 

The  apparent  changes  in  color  result  from  a  change  in 
interpretation.  In  the  erect  position  colors  are  attended 
to  as  signs  of  objects  of  certain  qualities  at  certain  dis- 
tances. In  the  inverted  position  the  perception  of  the 
latter  element  is  less  perfect,  and  the  colors  are  seen 
more  independently. 

The  nature  of  the  distance  illusion  has  been  differently 
reported  by  different  observers.  Helmholtz  and  others  see 
distant  objects  as  more  distant  with  the  head  erect ;  James 
sees  them  nearer.  Dr.  Washburn's  tests  with  pictures 
makes  it  probable  that  more  than  a  single  factor  may 
enter  at  times,  and  the  result  with  different  landscapes 
may  be  different.1  My  own  trials  suggest  that  with 
complicated  expanses  like  a  view  of  city  roofs  and  build- 
ings, which  is  poorly  disentangled  with  the  head  inverted, 
the  normal  position  gives  the  greater  distance ;  while  with 
a  simple  and  relatively  unbroken  expanse  the  reverse  may 
be  the  case. 

On  a,  Mach,  A,  50 ;  Filehne,  300  f. ;  Bering,  A,  571.  On  6,  Helm- 
holtz, A,  G.  607,  871  ff.;  Fr.  568  f.,  913,  915  (4332  723,  725);  Rood; 
James,  II.,  213;  Margaret  Washburn;  Filehne,  297  ff.;  Thiery,  102. 

GEOMETRICAL  ILLUSIONS. 

Beside  the  illusions  just  considered,  a  large  number  have 
been  found  that  affect  the  perception  of  plane  figures.  In 
some  of  these  the  effect  of  perspective  or  the  tendency  to 
interpret  the  lines  as  representations  of  three-dimensional 


1  Professor  James  does  not  say  explicitly  that  he  and  his  observers  were 
careful  about  keeping  the  eye  the  same  height  above  the  ground  in  the  two 
cases.  If  this  was  not  regarded  it  might  account  for  the  difference  in  his 
result. 


186]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         213 

figures  is  clear ;  in  others  the  influence  of  preponderating 
experience,  or  of  eye-movements,  is  to  be  observed.  It  is 
probable  that  many,  even  of  those  that  seem  most  sim- 
ple, are  the  resultant  of  several  simultaneous  tendencies ; 
considerable  individual  differences  in  the  extent  of  these 
illusions  may  therefore  be  looked  for.  A  full  treatment  is 
out  of  the  question  here,  the  brief  commentary  on  the  dia- 
grams being  intended  merely  as  a  suggestion  of  the  views 
held  with  regard  to  them,  not  as  an  exposition  or  criticism 
of  those  views.  The  student  will  do  well  to  turn  the  dia- 
grams about  and  to  view  them  from  different  sides,  so  as 
to  separate  the  illusions  that  depend  on  position  from  those 
that  do  not.  In  general,  illusions  are  strengthened  when 
the  affected  lines  are  made  oblique  in  the  field.  For  very 
exact  and  careful  study  the  diagrams  should  be  separated 
from  one  another  and  from  the  influence  of  all  extraneous 
lines,  e.  g.,  drawn  singly  on  good  sized  sheets  of  paper. 
Sometimes  the  size  of  the  diagrams  makes  a  difference, 
some  illusions  being  more  striking  in  large  figures,  others 
in  small.  The  figures  in  the  text  are  not  to  be  taken  as 
standards  in  this  regard.  Many  illusions  are  much  inten- 
sified when  the  figures  are  drawn  on  glass,  or  made  of  wire 
as  actual  models,  especially  when  their  parts  are  movable, 
and  the  extent  or  place  of  the  illusory  effect  can  be  changed 
before  the  eyes  of  the  observer.  Many  of  the  figures  in 
Bradley's  "  Pseudoptics  "  possess  this  advantage.1 


1  It  has  often  seemed  to  the  writer  that  many  illusions  showed  more 
strongly  in  rough  drawings  on  the  blackboard  or  on  paper,  even  when  the  figures 
were  so  made  as  to  throw  the  unavoidable  inaccuracies  against  the  illusory 
effect,  than  in  the  more  rigid  diagrams  of  the  books.  If  this  is  a  fact  it  might 
be  due  in  part  to  something  similiar  to  that  just  mentioned  —  the  actual  change 
of  the  figure  in  construction,  — and  in  part  to  the  greater  share  of  apperception 
in  such  rough  diagrams. 

For  illusions  depending  on  irradiation,  see  Chap.  VIII. 

On  the  Geometrical  Illusions  in  general,  see  Wundt,  A,  4te  Aufl.,  II.,  137- 
156;  Helmholtz,  A,  G.  705  ff. ;  Fr.  720  If.  (562  If.) ;  Hoppe,  A  ;  Lipps,  and  Thiery. 


214      LABORATORY  COURSE  IN  PSYCHOLOGY.     [187 

187.  The  Tendency  of  the  Eye  to  Follow  Lines  and 
Contours.  The  most  important  thing  in  ordinary  seeing  is 
a  clear  perception  of  form,  and  lines  and  contours  are  fol- 
lowed because  they  are  the  best  key  to  that  perception. 
Lines  that  are  followed  by  the  eye  (Wundt's  Fixation 
Lines)  are  more  clearly  seen  than  those  that  are  not,  and 
lines  lying  nearly  in  the  same  direction  as  those  that  are 
followed  are  favored  above  those  in  other  directions.  It  is 
easy  to  see  that  this  habit  of  the  eyes  must  play  a  great 
role  in  the  geometrical  illusions.  This  tendency  is,  how- 
ever, not  beyond  conscious  control,  and  for  that  reason  is 
more  difficult  to  demonstrate  by  overt  experimentation 
than  by  casual  observation.  Any  one  who  will  take  note  of 
his  own  seeing  when  presented  with  objects  with  strongly 
marked  lines,  will  easily  find  trace  of  the  habit.  In  ima- 
gining geometrical  figures  also  (for  example,  an  eighteen- 
inch  hexagon)  something  of  the  same  tendency  will  often 
be  found. 

a.  Let  the  observer  examine  the  figure  below  for  a  mo- 
ment, and  then  require  him  to  say  how  his  eyes  have 
moved  in  doing  so. 


He  will  probably  find  a  tendency  to  follow  the  middle 
line  of  the  left  figure,  and  the  vertical  lines  of  the  right. 

b.  Paste  upon  a  piece  of  cardboard  eight  and  one-eighth 
inches  long  and  four  inches  wide,  two  four-inch  squares  of 


188]        VISUAL   PERCEPTION  OF  SPACE,   ETC.         215 

red  paper  in  such  a  way  as  to  cover  all  the  card  except 
a  white  stripe  one-eighth  of  an  inch  wide  between  them. 
Cover  the  whole  with  a  sheet  of  semi-transparent  paper,  as 
for  Meyer's  experiment  (Ex.  152  c),  and  examine  the  white 
stripe  for  the  effects  of  contrast.  After  the  examination 
has  lasted  a  few  seconds,  suddenly  lay  across  the  middle 
of  the  diagram  a  bit  of  wire  six  or  eight  inches  long, 
at  right  angles  to  the  white  stripe.  If  the  experiment 
succeeds,  the  white  stripe  will  instantly  show  a  strong 
increase  in  the  complementary  color.  Before  the  introduc- 
tion of  the  wire,  the  eye  is  chiefly  engaged  in  following 
up  and  down  the  white  stripe,  and  the  contrast  effects  are 
confined  to  those  of  simultaneous  contrast.  When  the 
wire  appears,  the  eye  changes  to  it  and  moves  back  and 
forth  along  it  once  or  twice,  and  thus  brings  upon  the 
white  stripe  the  more  powerful  effects  of  successive  con- 
trast.1 

Thiery,  112  ;  Waller. 

188.  Perspective  Figures.  The  tendency  to  see  things 
spatially  is  so  inveterate  that  a  moderate  suggestion  of  per- 
spective is  sufficient  to  introduce  differences  in  apparent 
distance  and  so  of  apparent  size. 

a.  Yon  Bezold's  Figure.  The  following  figure,  after  von 
Bezold,  shows  the  tendency  in  question  sufficiently  well. 

It  is  interesting  to  observe  that  the  enlargement  of  the 
remoter  figures  is  not  so  great  as  the  represented  distance 
would  require  —  that  is,  the  perspective  interpretation  is 
not  effective  in  full  measure.  A  good  example  of  this  fig- 
ure will  be  found  in  Bradley's  "  Pseudoptics."  It  has  also 
been  used  in  advertising. 


1  This  experiment  originates  with  Waller  (Journal  of  Physiology,  XII.,  4, 
p.  xliv.),  but  is  used  by  him  for  a  different  purpose. 


216       LABORATORY  COURSE  IN  PSYCHOLOGY.     [189 


b.  Under  favorable  circumstances  the  perspective  illu- 
sion may  cause  an  apparent  distortion  of  an  actual  right 

angle. 

This  is  the  case  with  the 
lower  cross  on  the  left  of 
the  cube,  and  the  upper  cross 
on  the  right.  The  rectangu- 
larity  of  the  crosses  is  lost 
when  the  perspective  impres- 
sion is  strong,  and  vice  versa. 
The  same  effect  may  also  be 
observed  when  the  after-image 
of  a  rectangular  cross  is  pro- 
jected upon  a  drawing  of  a  cube.  Cf.  the  distortion  of  the 
rectangular  after-image  in  Ex.  131  b  (p.  124). 

On  a,  von  Bezold,  B  ;  on  6,  James,  II.,  254;  Thiery,  317. 

189.  Perspective  Interpretation  of  Plane  Figures.  Cer- 
tain arrangements  of  lines  tend,  upon  very  slight  sugges- 
tion, or  even  without  it,  to  take  on  a  three-dimensional 


189]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         217 


interpretation.  This  may  happen  with  oblique  lines  in 
drawings  on  paper,  but  is  easier  to  observe  with  wire 
models  or  figures  drawn  upon  glass,  where  there  is  less 
to  favor  a  plane  interpretation,  and  with  monocular  rather 
than  binocular  vision. 

Prepare  models  or  figures  on  glass  similar  to  those  below, 
place  them  in  such  a  position  that  they  may  be  observed 
against  the  sky  or  other  uniform  background,  and  observe 
with  a  single  eye. 


\ 


\f\ 


/ 


The  tendency  to  a  three-dimensional  interpretation  is 
stronger  in  A  and  B  than  in  C  and  D.  In  A  and  B  it 
is  the  oblique  lines,  not  the  vertical  and  horizontal  that 
are  affected,  at  least  to  an  appreciable  degree.  E  is  only  a 
reduplication  of  A,  but  the  perspective  interpretation  of 
the  short  lines  also  involves  a  slight  inclination  of  the 
verticals.  The  figures  are  almost  all  capable  of  two  per- 
spective interpretations ;  in  A,  for  example,  the  lower  end 
of  the  oblique  line  may  be  nearer  than  the  vertical  or  far- 
ther away.  (Of.  the  Equivocal  Figures  of  Exs.  201-203.) 

This  tendency  to  perceive  oblique  angles  as  perspective 
pictures  of  right  angles  is  perhaps  connected  with  the  ten- 
dency to  overestimate  small  angles,  and  underestimate 


218 


LABORATORY  COURSE  IN  PSYCHOLOGY.     [190 


large  ones,  long  familiar  to  those  who  have  busied  them- 
selves with  the  geometrical  illusions.     (Cf.  Ex.  190.) 
Hering,  A,  579  f.,  B,  79  f.;  Mach,  A,  96  ff. 

190.  Illusions  in  the  Perception  of  Angles.  Small  an- 
gles are  relatively  overestimated,  and  large  angles  relatively 
underestimated.1 


In  A  and  B  slight  distortions  are  found  in  the  horizontal 
lines.  In  C  the  circle  is  flattened  at  the  corners  of  the 
square,  and  the  sides  of  the  latter  are  bent  inward.  In  D, 
the  distortion  is  unmistakable,  but  probably  not  due  to  the 
small  angles  alone.  (Cf.  Exs.  192  and  195.)  E  seems  at 
first  to  contradict  the  principle  of  the  overestimation  of 
small  angles,  for  the  effect  is  the  same  in  kind  as  in  C, 
when  the  reverse  effect  might  have  been  expected.  But 
Thiery  is  probably  right  in  saying  (366  ff.)  that  the  im- 
portant condition  of  eye-movements  is  different  in  the  two 
cases.  In  C  the  eye-movement  seems  to  follow  the  chord  or 
the  arc  —  in  either  case  it  is  such  that  the  eye  meets  lines 
which  make  a  small  angle  with  the  one  along  which  it 


1  Jastrow  (A,  382)  considers  that  all  angles  are  underestimated,  both  acute 
and  obtuse,  but  the  obtuse  much  more  than  the  acute,  so  that  when  both  occur 
together,  as  they  commonly  do,  the  effect  is  equivalent  to  an  overestimation  of 
the  small. 


191]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         219 

moves.  In  E,  however,  when  the  eye  moves  along  one  of 
the  short  lines  toward  the  circle  it  tends  to  follow  on  along 
the  arc  that  would  require  the  least  change  of  direction, 
that  is,  the  one  with  which  it  makes  an  obtuse  angle.  If 
this  is  the  case,  the  illusion  is  one  of  obtuse  instead  of  acute 
angles,  and  the  result  is  in  accord  with  the  principle. 

The  general  case  of  the  overestimation  of  small  angles 
Wundt  refers  to  eye-movements  as  influenced  by  adjacent 
lines.  In  A,  for  example,  as  the  eye  follows  the  horizon- 
tal line  toward  its  intersection  with  the  oblique  lines,  it 
suffers,  as  it  were,  an  increasing  attraction  toward  the 
oblique  line  nearest  it,  and  from  this  results  the  wrong 
conception  of  its  route.  In  explanation  of  the  first  three 
figures,  Helmholtz  cites  his  principle  "  according  to  which 
acute  angles,  being  small  magnitudes  clearly  limited,  seem 
in  general  relatively  too  large,  when  we  compare  them  with 
undivided  obtuse  or  right  angles,"  but  believes  that  this 
principle  yields  in  importance  to  ocular  movements,  if, 
indeed,  it  does  not  itself  depend  on  them.  —  A,  G.  708  ff. ; 
Fr.  724  ff.  (566  ff.).  The  explanation  of  this  illusion  by 
the  tendency  to  see  oblique  angles  as  right  angles  in  per- 
spective has  already  been  mentioned. 

Thiery,  360  ff. ;  Wundt,  A,  4te  Aufl.,  II.,  145  ff. ;  Helmholtz,  A,  G. 
707  ff.;  Fr.  722  ff.  (564  ff.);  Hering,  A,  372  f.;  Mach,  A,  98;  Del- 
boeuf,  A;  Dresslar. 

191.    Zollner's  Figure. 

a.  In  this  well-known  figure  the  vertical  lines,  though 
actually  parallel,  appear  to  converge  alternately  above  and 
below.  They  may  also  show,  slightly,  the  same  tendency 
to  leave  the  plane  of  the  paper  that  is  sometimes  noticed 
in  the  long  lines  of  E,  Ex.  189,  of  which  this  figure  is 
simply  an  elaboration.  The  short  lines  also  show  the  illu- 
sion of  Poggendorff  (Ex.  193). 


220       LABORATORY  COURSE  IN  PSYCHOLOGY.      [191 

b.  The  illusion  is  considerably  increased  when  the  eyes 
move  across  the  diagram  at  right  angles  to  the  long  lines ; 
it  is  diminished  or  abolished  when  they  move  in  the  same 
direction  as  the  long  lines.  The  easiest  way  of  securing 
definite  movement  of  the  eyes  in  these  directions  is  to 


1 

II 

I 

I 

11 

1 

« 

5? 

I 

n 

\  / 

*! 

s  / 
s  / 
s  / 
s  / 
s  / 
s  ^ 

\\ 

L.            A 

\ 

J 

s 

s 
s 
s 

s 

s 
s 

s 

s 

s 

fixate  the  head  of  a  pin  moved  to  and  fro  across  the  dia- 
gram, or  to  move  the  figure  to  and  fro  behind  the  fixated 
pin-head.  A  certain  moderate  rate  of  movement,  easily  to 
be  found  by  trial,  gives  the  best  results.  Continuous  fixa- 
tion of  the  pin-head  is  very  important.  The  writer  finds 
some  help  in  bringing  the  diagram  rather  near  the  eye,  i.e., 
within  six  or  eight  inches.  Notice  with  the  crosswise 
movement,  beside  the  apparent  running  of  the  long  lines 
upward  and  downward  (which  will  be  considered  in  the  sec- 
tion on  the  visual  perception  of  motion),  their  inclination 
with  reference  to  the  plane  of  the  paper. 


191]        VISUAL   PERCEPTION   OF  SPACE,   ETC.         221 

c.  Several  other  points  with  regard  to  the  completeness 
of  the  illusion  have  been  observed.  Most  have  obvious 
relation  to  changes  in  the  movement  of  the  eyes  in  tra- 
versing the  figure.  Position  of  the  diagram :  If  the  dia- 
gram is  rotated  in  a  plane  parallel  to  the  frontal  plane  of 
the  face,  the  illusion  is  most  marked  when  the  long  lines 
make  an  angle  of  about  45°  with  the  vertical.  It  is  less 
when  these  lines  are  vertical,  and  less  also  when  they  are 
horizontal.  If  the  diagram  be  rotated  backward  from  the 
plane  just  mentioned  about  an  axis  parallel  to  a  line  con- 
necting the  centres  of  the  eyes,  the  illusion  will  be  de- 
creased if  the  long  lines  have  been  vertical  at  the  start,  and 
increased  if  they  have  been  horizontal.  The  size  and  dis- 
tance of  the  diagram  seem  to  affect  the  extent  of  the  illu- 
sion. (Kundt,  121,  148  ff.)  Transversals :  The  illusion  is 
usually  stronger  when  the  transversals  make  an  angle  of 
about  30°  with  the  long  lines,  but  may  reach  a  maximum 
at  a  smaller  angle  when  the  transversals  are  long.  (Hey- 
mans.)  It  is  said  to  increase  with  the  number  of  trans- 
versals up  to  a  certain  number,  but  then  to  decrease. 
Monocular  Vision  has  been  found  more  favorable  than  bi- 
nocular, and  wandering  of  the  eyes  than  steady  fixation. 
On  several  of  these  and  on  other  points  Zollner,  Kundt, 
Thiery,  and  Heymans  have  made  quantitative  experiments. 

Explanations  of  the  Zollner  figure  are  numerous.  It  is 
obviously  of  the  small  angle  type,  and  the  explanations 
given  for  the  other  figures  of  that  class  are  applied  to  this 
one  also.  Helmholtz  associates  with  the  overestimation 
of  small  angles  the  very  striking  upward  and  downward 
movements  of  the  long  lines  observed  in  experiment  b 
above.  Similar  movements  of  actual  objects  give  rise  to  a 
still  stronger  illusion  of  the  same  -sort.  (Cf.  Ex.  225.)  He 
indicates  also  how  the  whole  may  be  treated  under  the  gen- 
eral principle  of  contrast  —  here  a  contrast  of  direction. 


222       LABORATORY  COURSE  IN  PSYCHOLOGY.      [191 

Another  explanation,  originating  with  Volkmann l  and  sup- 
ported by  the  observations  of  others,  has  recently  been 
further  developed  by  Thiery.  It  is,  briefly,  that  even  in 
casual  observation  the  figure  is  seen  perspectively  —  not 
consciously,  but  in  effect.  The  short,  oblique  lines  repre- 
sent planes  seen  in  perspective,  each  pair  of  planes  form- 
ing a  roof-like  ridge,  and  each  ridge  inclining  forward  or 
backward,  as  the  case  may  be  ;  the  pair  at  the  left  of  the 
figure,  for  example,  form  a  ridge  that  inclines  forward,  the 
second  and  third  one  that  inclines  backward,  the  third  and 
fourth  forward,  and  so  on.  The  long  lines  are  projected  on 
these  inclined  surfaces,  and  are  therefore  interpreted  as 
converging  or  diverging  lines,  though  their  retinal  images 
still  remain  parallel,  exactly  as  parallel  retinal  images 
would  be  interpreted  if  they  had  originated  from  actually 
converging  or  diverging  lines  lying  in  sloping  surfaces.2 
Heymans  connects  the  figure  with  the  illusion  of  Melling- 
hoff  and  Loeb  (Ex.  197  d)  and  approves  its  tentative  class- 
ification as  a  case  of  contrast,  but  would  not  exclude  an 
ultimate  reference  to  vividly  conceived  eye-movements. 

d.   Variants  of  the  Zollner  Figure. 

The  figures  of  Pisco  and  Hering  are  chiefly  interesting 
historically.  The  checker-board  figure,  on  the  contrary,  is 
important  for  the  theory  of  the  illusion,  for  it  exhibits  the 
Zollner  effect  without  the  usual  short  oblique  lines.3  For 
the  discussion  of  this  and  other  figures  which  do  the  same, 
see  Heymans,  B. 


1  Physiologische  Untersuchungen  im  Gebiete  der  Optik,  Leipzig,  1863,  S.  163, 
cited  by  Hering,  A,  580. 

2  That  no  such  inclination  of  the  planes  is  seen  by  the  observer  until  it  is 
suggested  to  him  is  fully  recognized  and  even  insisted  upon  by  Thie"ry.    It  is  no 
more  necessary,  however,  that  the  perspective  factor  should  be  conscious  in 
order  that  it  may  influence  the  final  form  of  the  perception  than  that  the  par- 
tial tones  in  a  note  on  a  violin  should  be  consciously  recognized  before  it  can 
be  distinguished  from  a  note  of  the  same  pitch  on  a  flute.    Cf.  Thie'ry,  121 11". 

3  See  Notes  and  Suggestions,  p.  435. 


191]         VISUAL   PERCEPTION   OF  SPACE,   ETC,          223 


One  of  Hering's  Variants  of  the  Zollner  Figure 


Checker-board  Figure. 

Helmholtz,  A,  G.  708  ff.  ;  Fr.  723  ff.  (565  ff.)  ;  Bering,  A,  373, 
579,  B,  75  f.,  78  ff.  ;  Aubert,  A,  630  f  ;  Mach  A,  98  ;  Wundt,  A, 
4te  Aufl.,  II.,  144  ff.  ;  Zollner,  A  and  B  ;  Lipps,  B,  267  ff.  ;  Heymans, 
B  ;  Jastrow,  A  ;  Thiery,  312  ff.,  and  the  literature  cited  by  them. 

1  By  permission,  from  Ladd's  "Elements  of  Physiological  Psychology" 
(Copyright,  1887,  by  Charles  Scribner's  Sons). 


224       LABORATORY  COURSE  IN  PSYCHOLOGY.      [192 

192.    Figures  Based  on  Convergent  Lines.1 
a.    In  A  the  circles  are  of  equal  size,  but  that  next  the 
vertex  of  the  angle  seems  a  little  larger.     When  such  a 
figure  is  made  on  glass  the  apparent  difference  in  size  may 


1  In  many  of  these  figures,  though  there  is  little  or  no  conscious  tendency 
to  a  three-dimensional  interpretation,  the  lines  resemble  those  of  perspective 
drawings,  and  the  illusions  are  similar  in  character.  How  far  the  illusion  in 
any  case  is  due  to  perspective  suggestion  cannot  now  he  said.  In  the  figures  of 
paragraph  6,  it  can  hardly  be  detected  ;  in  those  of  a  and  c,  it  seems  to  be  pres- 
ent. If  a  distinctive  name  were  desired  for  such  figures,  they  might  be  called 
Perspectlform  Figures. 


192]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         225 

bring  about  an  apparent  difference  in  distance,  the  circle 
lying  most  remote  from  the  apex  of  the  angle  seeming  to 
lie  farther  back.  In  B  the  short  verticals  are  of  equal 
length,  but  that  at  the  left  seems  slightly  shorter.  For 
the  writer  this  illusion  is  rather  weak  and  evanescent. 
Such  effects  are  frequent  with  converging  lines.  For  quan- 
titative experiments  on  this  illusion  see  Thiery,  607  ff. 

b.    Trapezoids.     In  trapezoids  the  short  side  is  overesti- 
mated and  the  long  side  underestimated.     In  C  the  long 


side  of  the  left  figure  and  the  short  side  of  the  right  figure 
are  equal,  but  the  latter  seems  a  little  longer.  This  is  true 
also  of  equal  spaces  marked  off  on  the  longer  and  shorter 
sides  (see  F  below),  but  for  this  last  another  factor  may  be 
partly  responsible.  (Cf.  Ex.  197.) 


\ 


A  similar  effect  is  produced  on  figures  lying  outside  the 
trapezoid  next  the  short  side  or  the  long.  In  Z>,  for  exam- 
ple, the  trapezoids  are  exactly  alike;  but  the  upper  one 
(lying  next  the  long  side  of  the  lower)  seems  a  little 
smaller  than  the  lower  (lying  next  the  short  side  of  the 
upper).  In  E  the  same  is  true  of  the  open  distances 


226       LABORATORY  COUliSE  IN  PSYCHOLOGY.      [192 

between  the  ends  of  the  lines.  This  figure  also  shows  a 
possible  transition  to  Zollner's  figure,  all  that  is  neces- 
sary being  the  parallel  verticals. 

Figure  G  is  so  arranged  as  to  add  the  illusion  affecting 
the  sides  of  the  trapezoids  to  that  depending  on  their  po- 


sition. The  short  side  of  the  lower  figure  and  the  long 
side  of  the  upper  are  of  exactly  equal  length.  For  quanti- 
tative studies  of  these  figures  see  Thiery,  605  f.,  67  ff. 

An  examination  of  these  figures  shows  their  close  rela- 
tion to  the  Mtiller-Lyer  figure  of  Ex.  194. 


Her  ing's  Figure.1 


1  By  permission,  from   Ladd's  "Elements  of   Physiological  Psychology" 
(Copyright,  1887,  by  Charles  Scribner's  Sons). 


193]         VISUAL  PERCEPTION   OF  SPACE,   ETC.         227 

c.  In  the  two  figures  shown  here  the  perspective  sugges- 
tion is  sometimes  quite  strong,  the  points  of  convergence 
seeming  to  lie  far  in  the  background.  This  three-dimen- 
sional suggestion,  according  to  Thiery,  affects  the  parallels 
also,  making  some  parts  of  them  seem  more  remote  than 
others,  and  so  leading  in  those  parts  to  overestimation  of 
their  separation.  It  is  evident,  however,  that  the  figures 
can  be  classed  with  the  small  angle  group,  and  the  cause  of 


Wundt's  Figure. 

that  illusion  may  be  responsible  in  large  measure  for  the 
distortions  in  this  case.  Cf.  also  Mtiller-Lyer's  figure, 
Ex.  194  a. 

On  a,  Holtz  ;  Thiery,  607  ff.  On  6,  Miiller-Lyer,  A,  269  f. ;  Thiery, 
605  f.,  67  ff.  On  c,  Bering,  B,  74  ;  Thiery,  74  f. 

193.     Poggendorff's  Illusion.  • 

In  the  typical  figure  for  this  illusion,  A  below,  the  line 
on  the  left  is  the  real  continuation  of  the  lower  line  at  the 
right,  and  not  of  the  upper,  as  appears  to  be  the  case.  This 
illusion  is  strengthened  by  viewing  it  from  a  distance,  i.e., 
by  reducing  the  size  of  its  retinal  image.  It  is  weakened 
or  quite  destroyed  by  turning  the  figure  so  that  the  oblique 


228       LABOEATOEY  COURSE  IN  PSYCHOLOGY.     [193 

lines  are  vertical  or  horizontal.  The  length  of  the  oblique 
lines,  the  angle  at  which  they  cut  the  parallels,  and  the 
separation  of  the  parallels,  are  all  of  importance.  For 
quantitative  studies  see  Thie'ry,  357  ff.,  and  Burmester.  In 
B  the  three  oblique  lines  are  all  parts  of  one  straight  line, 
but  do  not  appear  so.1  C  is  a  figure  given  by  Wundt  to 


w 


/ 


show  that  the  presence  of  an  actual  oblong  is  not  necessary. 
The  illusion  in  C  is  feeble,  and  probably  is  different  in 
amount  for  different  observers ;  Burmester  found  it  entirely 
reversed  in  direction  (pp.  358,  390  f.).  It  may  even  be 
doubted  whether  the  illusion  in  this  case  really  belongs  to 


1  Figures  A  and  B  are  taken  by  permission  from  Ladd's  "  Elements  of  Physio- 
logical Psychology"  (Copyright,  1887,  by  Charles  Scribner's  Sons). 


194]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         229 

the  Poggendorff  type  and  is  not  rather  a  case  of  the  Mel- 
linghoff-Loeb  figure,  Ex.  197  d.  In  D  the  oblique  line  at 
the  left,  if  prolonged,  would  pass  through  the  point  of  in- 
tersection of  the  two  lines  at  the  right,  but  seems  to  lie 
too  low.  In  E  is  shown  the  increase  of  the  illusion  with 
decreasing  length  in  the  oblique  lines. 

Poggendorff's  figure,  according  to  Wundt,  involves  sev- 
eral illusions.  The  figure  is  estimated  too  large  in  the 
direction  of  its  prominent  (vertical)  lines,  and  in  this  the 
habitual  overestimation  of  verticals  helps.  When  the  latter 
is  excluded  by  turning  the  figure  on  its  side,  the  illusion 
that  remains  is  to  be  explained  partly  by  overestimation 
in  the  direction  of  prominent  lines,  and  partly  by  overesti- 
mation of  small  angles.  Helmholtz  suggests  irradiation  as 
a  co-operating  cause.  Thiery  connects  it  with  the  tendency 
to  see  inclined  lines  perspectively,  already  noticed  in  Ex. 
189. 

Helmholtz,  A,  G.  707  f . ;  Fr.  722  f.  (564  f.);  Hering,  A,  372; 
Wundt,  A,  4te  Aufl.,  II.,  148;  Delboeuf,  O;  Dresslar. 

194.  The  Figure  of  Miiller-Lyer.  This  is  by  far  the 
most  important  and  most  discussed  of  recent  figures. 

a.  The  common  form  of  the  figure  is  given  in  A.  The 
horizontal  lines  are  of  exactly  equal  length,  but  do  not 


seem  so.  These  figures  are  closely  related  to  those  of  Ex. 
192  b.  Both  a  and  b  may  be  conceived  as  composed  of  two 
trapezoids  having  a  side  in  common  (the  central  line),  and 
a  side  omitted.  In  a  the  short  sides  are  common  and  the 
long  omitted  ;  in  b  the  reverse.  They  may  even  be  derived, 


230       LABORATORY   COURSE  IN  PSYCHOLOGY.      [194 

though  somewhat  arbitrarily,  from  the  figures  of  Ex.  192  c ; 
a  may  be  reached  from  Hering's  figure  by  extending  the 
-point  of  convergence  into  a  horizontal  line  and, reducing  the 
converging  lines  to  four  j  b  from  Wundt's  figure  by  taking 
any  four  connected  lines  forming  a  lozenge  and  supplying 
the  horizontal.  Miiller-Lyer  himself  connects  the  figure 
with  the  illusion  affecting  the  apparent  length  of  the  arms 
of  angles  noticed  in  Ex.  199  d. 

b.  A  Number  of  Variants  of  the  Muller-Lyer  figure  are 
given  in  the  accompanying  cut. 

The  variations  in  A  are  intended  to  show  that  the  illusion 
persists  when  the  central  lines  are  omitted;  when  the  oblique 
lines  do  not  actually  touch  the  central  lines ;  and,  in  some 
degree,  at  least,  when  no  oblique  lines  are  used.  B  is  a 
group  of  geometrical  figures,  the  first,  second,  fourth,  and 
fifth  of  which  involve  a  part  of  the  lines  of  the  typical  fig- 
ure. The  upper  half  of  the  hexagon,  for  example,  may  be 
regarded  as  a  of  the  typical  figure  with  the  upper  oblique 
lines  removed,  and  the  upper  half  of  the  next  figure  as  b 
of  the  typical  figure  treated  in  the  same  way.  The  central 
rectangle  is  added  for  purposes  of  comparison.  In  all  the 
figures  the  horizontal  lines  at  the  top  and  bottom  are  equal. 
If  the  fine  horizontals  are  taken  as  central  lines,  the  first 
and  fifth  figures  become  examples  of  b,  and  the  second  and 
fourth  of  a,  in  the  typical  figure.  C  shows  the  illusion  pres- 
ent when  curved  lines  are  substituted  for  the  oblique  lines  of 
the  typical  figure.  The  figures  from  D  to  G  show  the  effect 
of  changes  in  the  typical  figure  itself.  In  D  the  horizontals 
are  equal,  but  the  excessive  lengthening  of  the  oblique  lines 
has  weakened  the  illusion,  —  according  to  Mtiller-Lyer,  be- 
cause of  a  strong  contrastive  effect ;  cf.  Ex.  197.  In  E  the 
figures  are  alike,  except  in  the  angle  made  by  the  oblique 
lines.  The  angles  between  the  obliques  are  approximately 
120°,  90°,  60°,  30°,  and  15°.  In  F  all  the  horizontals  are 


194]         VISUAL  PERCEPTION  OF  SPACE,   ETC.          231 


A 


V 


V 


A 


111 


U 


n          i 


/\ 


\/ 


\/ 


A 


-c 


V 


\l          \l 


r                       1 

SL                                / 

/                                    \ 
\                                     / 

f                                     \ 
\                                     / 

/ 

A 


232       LABORATORY  COURSE  IN  PSYCHOLOGY.      [194 

equal,  and  the  figures  show  the  usual  shortening  and  length- 
ening with  reference  to  the  standard  lines  beside  them.  If, 
however,  the  extent  of  the  illusion  in  the  two  cases  is  re- 
garded, it  is  found  to  be  greater  in  the  second  case ;  i.e.,  in 
a  of  the  typical  figure.  G  shows,  in  the  right-hand  figure, 
a  weakening  of  the  illusion,  due  to  the  omission  of  half  of 
the  oblique  lines.  Quantitative  studies  have  been  made  by 
Mtiller-Lyer,  Auerbach,  Heymans,  Binet,  van  Biervliet,  and 
Thiery  (73  ff.). 

Explanations  of  the  Figure  of  Miiller-Lyer.  Many  ex- 
planations of  this  figure  have  been  offered.  Mtiller-Lyer 
himself  regards  the  overestimation  in  one  case,  and  under- 
lestimation  in  the  other,  as  effects  of  the  areas  inclosed  by 
•the  sloping  lines  above  and  below  the  horizontal.  In  a  of 
the  typical  figure  these  are  longer  than  the  line,  and  cause 
it  to  seem  longer ;  in  b  they  are  shorter,  and  cause  the  line 
also  to  seem  shorter.1  In  the  fourth  figure  of  A  the  added 
parallels  take  the  place  of  the  spaces.  Auerbach.  holds  sub- 
stantially the  same  opinion,  and  that  of  Jastrow  is  not  very 
different. 

Thiery  considers  this  figure  an  elaboration  of  the  trape- 
zoid,  the  illusion  being  strengthened  by  duplication  above 
and  below.  The  illusion  would  then  be  one  of  the  perspec- 
tiform  class,  and  the  apparent  difference  in  the  length  of 
the  lines  would  rest  ultimately  on  a  difference  of  the  dis- 
tances to  which  they  are  (unconsciously)  referred.  Thiery 
finds  a  ground  for  this  difference  of  the  distance  factor  in 
the  different  ways  in  which  the  eyes  traverse  the  two  figures. 


1  If  this  explanation  is  true,  we  evidently  have,  not  a  contrast  effect,  but  its 
opposite  —  an  adjuvant  effect  —  exercised  by  one  perceptive  quantity  on  another. 
Illusions  of  this  kind  Miiller-Lyer  calls  "Illusions  of  Confluence."  Under 
other  circumstances,  as  in  Ex.  197,  he  considers  that  an  actual  contrast  is 
present.  In  D,  above,  the  confluent  and  contrastive  tendencies  are  both  pres- 
ent. In  the  short-armed  figure  they  co-operate;  in  the  long-armed  they  are 
opposed.  (Miiller-Lyer,  Ji,  11  ff.,  f,  423  f.) 


195]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         233 

Some  of  the  other  explanations  may  be  mentioned  still 
more  briefly.  Delboeuf  and  Wundt  have  held  that  the  eye, 
in  moving  along  the  central  lines,  tends  to  follow  out  upon 
the  short  lines  when  they  are  outwardly  directed,  and  to 
stop  short  of  the  end  of  the  central  line  when  they  are  in- 
wardly directed.  Heymans  also  holds  an  eye-movement 
theory,  but  of  somewhat  different  form.  In  discussing 
these  and  other  geometrical  illusions,  Lipps  makes  use  of 
many  aesthetic  forms  of  expression,  such  as  "liveliness," 
" inner  activity,"  "upward  striving,"  thus  seeming  to  at- 
tribute the  illusory  effect  to  "  forces "  inherent  in  the 
figures.  His  thought  is  rather,  however,  that  the  illusion 
depends  on  the  meaning  of  the  figures  as  perceived  things. 
Eye-movements  are  influential,  but  the  conception  of  the 
figures  controls  them.  Cf.  Lipps,  B,  284  fT.  Brjmfit  has 
held  that  these  figures  are  like  those  of  Ex.  199  c,  and 
that  we  do  not  really  estimate  the  distance  from  the  apex 
of  the  short  lines,  but  from  the  centre  of  one  group  of 
short  lines  to  the  centre  of  the  other,  as  if  the  pair  formed 
a  triangle  with  an  imaginary  side.  The  skilful  observer 
will  very  likely  find  several  of  these  tendencies  in  his  own 
study  of  the  figure,  and  the  explanations  are,  indeed,  not 
totally  exclusive  of  one  another. 

Miiller-Lyer,  A,  B,  and  C.  Jastrow,  A,  396.  Brentano.  Delboeuf, 
C.  Wundt,  A ,  4te  Aufl. ,  II. ,  149  f .  Brunot ;  Auerbach ;  Heymans,  A ; 
Binet;  Thiery,  67  ff . ;  van  Biervliet. 

195.    Illusions  of  Interrupted  Extent. 

Interrupted  intervals  generally  seem  larger  than  free 
intervals.  In  the  first  two  figures  above,  the  interrupted 
extents  seem  larger  than  the  free  extents.  In  E,  however, 
where  the  interrupted  space  has  but  a  single  dot  in  the 
middle,  the  principle  suffers  an  exception.  B  should  be 
regarded  binoculaiiy,  for  monocular  observation  introduces 


234      LABORATORY  COURSE  IN  PSYCHOLOGY.      [195 

an  illusion  affecting  the  perception  of  the  vertical.     C  and 
D  are  equal  and  square.1 

Wundt's  explanation  rests  on  variation  in  eye-movements. 
In  passing  over  interrupted  extents,  the  movement  of  the 
eye  is  made  more  difficult  by  the  short  stages  into  which 
its  course  tends  to  break,  while  it  sweeps  with  relative 


freedom  over  the  empty  spaces.  The  fact  that  a  single 
interruption  in  the  middle  of  a  space  has  an  opposite  effect, 
is  explained  by  a  tendency  of  the  eye,  when  the  middle  of 


1  The  illusion  in  these  squares  runs  counter  to  the  practice  of  dressmakers, 
who  recommend  vertical  stripes  as  a  means  of  increasing  the  apparent  height. 
The  explanation  of  the  contradiction  seems  to  lie  in  the  difference  of  the  use  of 
the  eyes  in  the  two  cases.  In  comparing  the  squares  in  height  or  breadth  the 
eye  is  forced  to  move  across  the  lines  of  one  or  the  other,  and  the  illusion  of  in- 
terrupted extent  is  thus  introduced.  In  looking  at  a  garment,  however,  the  eye 
follows  the  most  prominent  lines,  and  tends  not  to  cross  them.  In  a  vertically 
striped  dress,  therefore,  it  is  the  length  of  the  vertical  lines  that  is  the  chief 
element  in  perception.  It  is  probable  that  in  a  garment  with  few  and  strongly 
marked  transverse  stripes  the  eye  tends  to  move  transversely  rather  than  verti- 
cally, and  the  breadth  of  the  wearer  is  more  regarded  than  the  height.  If,  how- 
ever, the  transverse  stripes  are  very  numerous,  the  eye  may  follow  the  general 
outlines  of  the  figure  instead  of  the  single  stripes,  and  an  overestimate  of  the 
height  again  result  from  the  interruption  of  the  extent.  . 


196]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         235 


an  extent  is  marked,  to  take  in  the  extent  simultaneously 
by  fixation  of  the  middle  without  motion.  For  other  ex- 
planations see  Helmholtz,  A,  G.  705  f.  ;  Fr.  720  f.  (562  f.), 
and  Loeb,  C.  For  quantitative  studies  see  Kundt,  128  ff. ; 
Aubert,  B,  264  ff.  ;  Messer ;  and  Knox  and  Watanabe. 

Hering,  J5,  65  ff.;  Wundt,  A,  4te  Aufl.,  II.,  142  f.;  and  the  litera- 
ture just  cited. 

196.  Illusions  in  the  Perception  of  Distances  Depending 
on  their  Direction  in  the  Field  of  Vision. 

a.  Vertical  distances  tend, to  be  overestimated  in  com- 
parison with  horizontal  distances.  Lay  off  by  eye  on  a 
sheet  of  paper  placed  vertically  before  the  face  equal  dis- 
tances, up,  down,  right  and  left,  from  a  central  dot,  mark- 
ing the  distances  by  dots  as  in  A  in  the  accompanying 
diagram.  Repeat  several  times,  and  measure  the  distances 
found.  The  illusion  is  said  to  be  more  marked  in  large 
figures. 

In  all  the  figures  below,  the  vertical  and  hprizontal  dis- 


236       LABORATORY  COURSE  IN  PSYCHOLOGY.     [196 

tances  are  equal,  and  in  all  cases,  except  in  the  circle,  the 
vertical  are  apt  to  seem  too  great.1 

In  D  the  overestimation  of  the  height  entails  an  under- 
estimation of  the  angles  above  and  below,  and  an  overesti- 
mation of  those  at  the  right  and  the  left.  In  E  the  base 
and  the  altitude  are  equal,  though  the  latter  seems  greater. 
The  same  illusion  causes  equilateral  triangles  to  seem  only 
isosceles.  Thiery  finds  in  the  latter  case  (p.  116  f.)  that 
the  effect  is  not  absolutely  dependent  on  the  vertical,  but 
that  any  side  taken  as  a  base  seems  too  short.  The  illusion 
is  less  in  the  line  figures,  and  absent  entirely  in  the  circle, 
in  Wundt's  opinion,  because  in  the  case  of  these  familiar 
figures  perception  is  influenced  by  knowledge  of  the  geo- 
metrical relations  of  the  parts ;  in  Thiery's  opinion,  because 
the  tendency  to  give  them  a  perspective  interpretation  with 
the  upper  part  remote  is  less  developed  (see  c  below).  A 
similar,  though  slight,  difference  is  sometimes  found  be- 
tween horizontal  distances  to  the  right  and  left,  when 
careful  experiments  are  made  with  the  single  eye.  For 
quantitative  studies  of  such  illusions  as  these,  see  Kundt, 
E.  Fischer,  Mtinsterberg,  and  the  literature  cited  by  them. 

b.  Distances  in  the  upper  part  of  the  field  are  over- 
estimated as  compared  with  those  below  them.  This  illu- 
sion may  be  tested  actively  as  follows  :  Near  the  top  of  a 
strip  of  paper  twelve  or  fifteen  inches  long  and  five  or  six 
wide,  draw  a  horizontal  line  about  two  inches  long.  Take 
this  as  a  standard,  and  draw  half  an  inch  below  it  a  second 
line  of  a  length  that  seems  equal  to  the  first.  Then  cover 


1  A  similar  illusion  attends  the  vision  of  many  distant  objects.  Photographs 
with  hills  in  the  background  are  often  disappointing,  because  the  hills  seem 
so  much  lower  in  the  picture  than  in  the  actual  landscape.  An  interesting  test 
may  be  made  by  requiring  a  number  of  persons  to  make  sketches  of  such  a 
landscape,  and  then  comparing  with  a  photograph  taken  on  the  spot.  Cf. 
Helmholtz,  A,  (1.  706  f.;  Fr.  722  (5G4). 


196]         VISUAL   PERCEPTION  OF  SPACE,    ETC.         237 

the  first  line,  and  taking  the  second  as  a  standard,  draw  a 
third,  and  so  on,  continuing  this  process  till  the  strip  is  full. 
Then  uncover,  and  measure  the  first  line  and  the  last.  With 
this  Wundt  associates  the  S's  and  8's,  which  seem  a  little 
smaller  at  the  top  than  at  the  bottom  when  in  their  usual 
position,  but  a  good  deal  larger  above  when  inverted,  —  S  8. 
For  similar  observations  on  other  letters,  see  Thiery,  p. 
97  ff. 

c.  The  overestimation  of  distances  in  the  upper  part  of 
the  field  seems  to  result  from  an  unconscious  allowance  for 
greater  distance  in  objects  lying  in  that  part  of  the  field. 
We  habitually  see  carpet  patterns,  figures  in  tablecloths, 
and  similar  repeated  figures  lying  in  planes  parallel  to  the 
floor,  as  of  equal  size  regardless  of  their  distance.  That 
there  is  a  tendency  to  such  an  allowance  may  be  shown  by 
the  following  experiment :  Prepare  a  frame  a  foot  or  more 
square,  and  stretch  across  it  a  set  of  six  or  eight  parallel 
threads  an  inch  apart.  Hold  the  frame  parallel  to  the  face 
before  a  uniform  background,  and  then  tilt  it  slowly  back- 
ward from  the  face  till  the  threads  begin  to  seem  conver- 
gent above.  Notice  the  angle  of  tilting,  and  then,  return- 
ing to  the  vertical  position,  tilt  the  frame  toward  the  face 
until  the  threads  seem  to  converge  below.  It  will  gen- 
erally be  found  that  the  angle  is  smaller  in  the  second 
case,  unless  special  effort  is  made  to  discover  the  conver- 
gence. 

For  all  of  the  illusions  of  a  and  b,  Wundt  finds  an  ex- 
planation in  the  differences  of  effort  required  for  turning 
the  eye  in  different  directions.  The  superior  and  inferior 
straight  muscles  are  weaker  than  the  external  and  internal. 
Thiery's  explanation  also  depends  upon  eye-movements,  but 
in  a  different  way.  Depression  of  the  plane  of  vision  is 
accompanied  by  increased  convergence,  and  this  is  associ- 
ated with  vision  of  near  objects.  The  lower  part  of  a  fig- 


238       LABORATORY  COURSE  IN  PSYCHOLOGY.     [197 

ure  is  therefore  likely  to  be  interpreted  as  if  nearer  than 
the  upper  part.  There  is  also  a  tendency  to  run  the  eye 
over  such  figures  from  below  upward,  which  also  favors 
the  interpretation  of  the  lower  parts  as  nearer  than  the 
upper.  Lipps  regards  the  overestimation  of  the  height  of 
the  square  as  an  unconscious  allowance  for  foreshortening, 
acquired  through  preponderating  experience  with  squares 
lying  in  planes  inclined  with  regard  to  the  plane  of  vision ; 
and  Bering's  explanation  of  the  results  of  Ex.  c  is  the 
same. 

Kundt.  Miinsterberg,  164  f.,  175  ff.  Wundt,  A,  4te  Aufl.,  II.,  137 
ff.  Bering,  B,  355  f.  Helmholtz,  A,  G.  684,  702;  Fr.  697,  716  (543, 
559).  Thiery,  93  ff.  Lipps,  B,  221. 

197.    Contrastive  Illusions. 

a.  In  figures  like  the  first  and  second  groups  below,  the 
middle  space,  angle,  line,  or  circle  seems  smaller  when  it 
lies  between  large  extents  than  when  it  lies  between  small 
extents. 


The  figures  of  the  first  group  were  introduced  by  Miiller- 
Lyer,  the  circles  by  Ebbinghaus.     With  these  probably  be- 


197]         VISUAL   PERCEPTION    OF  SPACE,    ETC.         239 


O 
00 

o  ^-^  o 

O 


longs  the  following  figure,  devised  and  studied  quantita- 
tively by  Baldwin.  The  bar  is  exactly  half-way  between 
the  circles,  but  looks  a  little  nearer  the  larger  one. 


Muller-Lyer  seems  to  hold  that  the  observer  compares 
the  extents  in  question  with  their  surroundings  as  well  as 
with  each  other.  Thiery  cites  from  Classen  the  general 
statement  that  "the  larger,  longer,  higher,  broader  the 


240       LABORATORY    COURSE  IN  PSYCHOLOGY.     [197 

mathematical  form  of  an  object  appears,  the  more  we  have 
reason  to  regard  it  as  near ;  the  smaller,  shorter,  lower,  nar- 
rower, the  more  we  imagine  it  at  a  distance  ; "  and  argues 
that  the  figures  of  the  group  of  large  extent  are  interpreted 
as  they  would  be  if  less  remote  than  the  group  of  small  ex- 
tent, from  which  fact  results  the  apparent  difference  in  the 
parts  of  the  figures  which  are  actually  equal. 


b.  In  a  similar  way,  the  sides  and  bases  of  parallelograms 
seem  to  contrast,  the  tall  figure  appearing  narrower  than 
the  shorter;   and  even  simple  parallel   lines   seem  nearer 
together  or  farther  apart  as  they  are  longer  or  shorter.     In 
this  case,  which  is  the  one  specially  noticed  by  him,  Wundt 
explains  the  illusion  by  the  strong  tendency  to  move  the 
eyes  lengthwise  along  the  long  parallels,  which  results  in 
an  underestimation  of  the  figure  in  a  vertical  direction. 

c.  Contrast  in  Curvature.     In  the  following  figures  from 
Lipps  and  Hofler,  curved  lines  make  adjacent  straight  lines 
seem  curved  in  the  contrary  direction. 

In  A  there  is  a  slight  apparent  curvature  of  the  straight 
line  connecting  the  spirals.  In  B  the  lines  for  a  half-inch 
either  side  of  the  apex  are  actually  straight,  but  seem 
slightly  convex  downward.  In  C  the  last  half-inch  on 
either  side  is  straight,  but  seems  slightly  curved  in  the 
same  sense  as  in  B. 


197]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         241 


d.  The  illusion  of  Melliiighoff  and  Loeb. 

In  A  the  lower  line  of  the  right  pair,  and  the  upper  line 
of  the  left,  are  parts  of  the  same  straight  line,  but  the  addi- 
tion of  the  parallels  causes  the  first  to  seem  a  little  too  low, 
the  second  a  little  too  high.  The  illusion  is  said  to  be 
lessened  when  attention  is  withdrawn  from  the  added  par- 
allels. Loeb  gives  the  experiment  an  active  form  by  hav- 
ing strips  of  cardboard  laid  somewhat  as  in  this  diagram, 


but  has  the  head  fixed,  and  tke  strips  laid  at  one  side  of 
the  median  plane.  He  also  gives  quantitative  results  in 
round  numbers.  Hey  mans  (J5,  120  if.)  gives  quantitative 
results  for  the  median  position.  B  is  the  figure  of  Mel- 
linghoff  (Wundt,  A,  4te  Aufl.,  II.,  146).  The  dots  are  really 
at  the  level  of  the  lower  line,  but  seem  a  little  too  high, 


242       LABORATORY  COURSE  IN  PSYCHOLOGY.     [198 

especially  when  the  figure  is  held  so  as  to  make  the  lines 
oblique.  Loeb  refers  the  illusion  to  spatial  contrast; 
Wundt  (in  discussing  MellinghofF s  figure)  to  the  effect  of 
the  added  parallels  on  the  movements  of  the  eyes  in  trav-' 
ersing  the  diagram.  It  seems  possible,  however,  that  these 
views  need  not  exclude  each  other. 

Muller-Lyer,  A,  B,  and  C;  Thiery,  83  ff.;  Wundt,  A,  4te  Ann., 
II.,  146  f.;  Baldwin;  Aubert,  A,  629;  Lipps,  B,  300;  Hofler;  Loeb,  C. 

198.  Contour  Illusions.  When  one  side  of  a  quadrilat- 
eral figure  is  removed,  the  figure  seems  too  long  in  the  direc- 
tion toward  the  side  removed,  and  too  short  in  the  other 
direction.  In  the  figure  below,  the  three-sided  squares  seem 
too  long  in  a  horizontal  direction,  too  short  vertically.  The 
reverse  is  the  case  with  the  space  between  them,  which  is 
also  a  square  of  equal  size. 


The  explanation  given  by  Mtiller-Lyer  for  this  illusion 
is  double.  The  open-sided  squares  seem  too  long  in  the  di- 
rection of  the  open  side,  because  a  certain  portion  of  the 
free  space  is  included  in  the  estimate  (illusion  of  conflu- 
ence) ;  and,  like  the  parallelogram  of  Ex.  197,  they  seem 
too  small  in  the  other  direction  because  they  seem  too  long 
in  this. 

When  the  circumference  of  a  circle  is  interrupted,  the 
remaining  arcs  seem  too  flat  to  belong  to  a  circle  of  such 
radius ;  so  also  a  semicircle  seems  like  an  arc  of  a  greater 


198]         VISUAL  PERCEPTION   OF  SPACE,   ETC.         243 

circle  of  less  than  180°  extent.     Closing  it  by  drawing  the 
diameter  makes  it  seem  smaller. 


This  illusion  Mtiller-Lyer  connects  with  his  typical  fig- 
ure by  a  transition  through  the  next  figure  below,  the  chief 
difference  being  that  the  central  line  is  now  curved  instead 
of  straight. 

The  short,  straight  lines  tend  to  shorten  and  bend  one  arc, 


and  lengthen  and  flatten  the  other.  If  short  arcs  of  the 
same  radius  as  the  central  ones  are  substituted  for  the  short, 
straight  lines,  and  then  are  rotated  till  they  form  a  contin- 


244      LABORATORY  COURSE  IN  PSYCHOLOGY.     [199 

uation  of  the  middle  arcs,  they  will  still  produce  the  usual 
effect.  From  this  would  result  the  general  principle,  that 
every  portion  of  an  arc  influences  the  form  of  every  other 
portion,  in  the  direction  of  contraction  in  the  complete  circle 
and  large  arcs,  and  of  expansion  in  arcs  of  less  than  180°. 
It  is  clear  to  casual  observation  that  the  diameter  of  a  circle 
looks  shorter  than  the  side  of  a  square  of  equal  breadth. 


Wundt  holds  that  the  small  arcs  seem  flat,  because  the 
movement  of  the  eye  in  following  them  is  not  very  remote 
from  that  for  following  a  straight  line. 

Miiller-Lyer,  A  and  (7;  Wundt  A,  4te  Aufl.,  II.,  149,  152  ;  Lipps, 
B,  233  f.,  290. 

199.  Miscellaneous  Geometrical  Illusions.  Under  this 
head  are  gathered  a  number  of  illusions,  some  of  which  can 
be  explained  more  or  less  easily  as  variants  of  forms  already 
considered,  others  that  can  only  be  brought  into  line  with 
difficulty,  and  still  others  which  depend  on  clearly  different 
causes. 

a.  King  Segments.  In  both  A  and  B  the  upper  segment 
seems  smaller,  though  all  are  of  precisely  the  same  size. 
The  illusion  is  even  more  striking  when  the  segments  are 
cut  out  of  cardboard,  and  can  be  shifted  about  and  actually 
superposed.  This  illusion  is  probably  a  special  case  of  the 
trapezoid  illusion  (Ex.  192  £),  and,  like  that,  is  related  to 
the  Zollner  figure,  the  transition  to  which  is  shown  in  (7,  Z>, 


199]          VISUAL  PERCEPTION  OF  SPACE,   ETC.         245 

and  E.  Wundt  denies  the  explanation  based  on  this  rela- 
tion to  the  Zollner  figure,  asserting  that  if  this  were  the 
case,  the  effect  ought  to  be  the  reverse,  i.e.,  the  upper  seg- 
ment should  seem  larger,  and  gives  in  proof  F9  in  which  the 
upper  curves  seem  a  little  larger ;  but  he  evidently  has  only 
considered  the  curves,  and  not  the  straight  lines  at  the  ends 
of  the  segments.  His  own  explanation,  while  regarding 


\ 
\ 
\ 
\ 
\ 


eye-movements  as  a  primary  cause,  lays  considerable  stress 
on  the  reference  of  the  segments  to  the  same  centre. 

b.  Concentric  Circles.  The  inner  circle  in  the  figure  at 
the  left  and  the  outer  in  the  figure  at  the  right  are  of 
exactly  equal  size,  but  the  latter  seems  smaller. 

In  the  opinion  of  Delboeuf,  who  introduced  the  figure, 
the  illusion  depends  on  the  interference  of  the  extra  circles 
with  the  measurement  of  the  diameter  by  the  eye.  In  the 


246       LABORATORY  COURSE  IN  PSYCHOLOGY.      [199 

right  figure  the  inner  circle  holds  the  eye  back,  as  it  were, 
and  in  the  other  the  outer  circle  draws  it  on.  Wundt's  ex- 
planation is  here,  as  in  the  case  of  the  parallel  lines  of  Ex. 
197  b,  the  underestimation  of  distances  in  directions  op- 
posed to  the  chief  tendency  to  movement.  The  eyes  tend 
to  follow  the  parallel  circumferences ;  and  this  causes  under- 


estimation of  the  distance  between  them,  making  the  larger 
seem  too  small  and  the  smaller  too  large.  If  this  tendency 
to  movement  is  opposed  by  a  fixation  mark  in  the  centre, 
he  finds  that  the  illusion  disappears,  as  in  the  case  of  the 
space  broken  by  a  single  central  dot  in  Ex.  195.  Thiery 
offers  a  perspective  explanation,  and  Loeb  attempts  one 
resting  on  differences  in  accommodation. 


o o  e-e 


c.  Dumbbell  Figures.  In  this  figure,  also  from  Del- 
boeuf,  the  distance  between  the  adjacent  edges  of  the  left 
pair  of  circles  is  the  same  as  that  between  the  remote 
edges  of  the  right  pair,  though  the  latter  looks  consider- 
ably less.  The  illusion  has  been  given  an  active  form 


199]         VISUAL   PERCEPTION   OF  SPACE,   ETC.         247 

(Hopkins)  by  placing  three  like  coins  close  together  in  a 
row,  and  then  requiring  that  the  middle  one  be  moved 
away  at  right  angles  from  the  others  till  the  distance 
between  its  edge  and  that  of  either  of  the  others  shall 
be  equal  to  the  distance  of  the  remote  edges  of  the  other 
two.  This  has  been  done  exactly  in  the  right  figure  below, 
but  to  most  observers  the  distance  will  seem  too  great.  It 
will  also  be  noticed  that  the  free  space  between  the  adja- 
cent circles  in  the  right  figure  seems  larger  than  the  middle 
circle  in  the  left  figure. 


o 


The  illusion  belongs  to  the  type  of  Miiller-Lyer's  figure ; 
and  Delboeuf's  explanation,  as  for  that  and  for  the  con- 
centric circles,  is  that  in  one  figure  the  eye  is  drawn  on- 
ward beyond  the  central  line,  and  in  the  other  falls  short. 

d.  Illusions  Affecting  the  Sides  of  Angles.  In  A  the 
oblique  arms  of  the  two  angles  are  of  equal  length,  but  that 
of  the  obtuse  angle  seems  longer.  In  B,  however,  though 
the  oblique  arms  are  again  equal,  that  belonging  to  the 
smaller  angle  seems  longer.  A  originates  with  Miiller- 
Lyer,  who  would  explain  the  illusion  on  the  same  principle 
as  his  typical  figure,  parts  of  which  these  may  be  conceived 
to  be.  B  was  devised  by  Laska  as  a  counter  argument. 
Miiller-Lyer's  reply  is,  that  the  character  of  the  figure  is 


248       LABORATORY  COURSE  IN  PSYCHOLOGY.     [199 

such  as  to  give  the  effect  of  another  (imaginary)  line,  that, 
namely,  which  would  connect  the  free  ends  of  the  oblique 
lines.  Such  a  line  would  form  an  obtuse  angle  with  the 
oblique  line  on  the  right,  an  acute  angle  with  that  on  the 
left,  and  consequently  tend  to  lengthen  the  first  and  shorten 


the  second,  which  would  cause  the  reversal  of  the  illusion. 
C  is  a  figure  used  by  Lipps  against  Brentano's  explanation 
of  Miiller-Lyer's  typical  figure,  and  is  interesting  in  several 
particulars.  All  the  lines  except  those  that  end  free  are 


199]        VISUAL   PERCEPTION   OF  SPACE,   ETC.         249 

of  equal  length;  the  four  that  end  free,  though  shorter 
than  the  rest,  are  equal  among  themselves  ;  the  angles 
between  the  oblique  lines  are  right  angles.  All  the  lines 
adjacent  to  obtuse  angles  appear  too  long,  those  adjacent 
to  the  acute  angles  too  short,  and  the  central  horizontal 
seems  longer  than  the  equal  space  between  the  inward 
turned  ends  of  the  upper  pair  of  short  lines. 


e.  In  this  figure,  from  Lipps,  the  five  central  lines  are 
equal  and  parallel,  but  those  with  the  short  horizontal  arms 
seem  longer  and  more  nearly  horizontal  than  the  others. 
Lipps  uses  the  figure  against  the  universality  of  the  prin- 
ciple of  the  underestimation  of  obtuse  angles  ;  but  to  the 
writer,  the  lack  of  parallelism  seems  due  rather  to  a  ten- 
dency to  judge  the  direction  from  the  trend  of  the  line 
groups  as  wholes,  instead  of  from  the  particular  lines  to 
be  compared.     The  difference  in  the  length  of  the  lines 
would   seem   related   to  the  contrastive   effect  shown   in 
Ex.  194  b,  Fig.  D. 

f.  In  the  figure  at  the  left,  though  both  lines  are  equal, 


250      LABORATORY  COURSE  IN  PSYCHOLOGY.     [199 

the  vertical  seeins  longer,  as  in  the  figures  of  Ex.  196.  In 
the  figure  at  the  right,  however,  the  vertical  seems  shorter, 
apparently,  because  of  the  single  division  of  its  length,  as 
in  Fig.  E  of  Ex.  195. 


B 


g.  Effect  of  Prominent  Lines.  The  figure  at  the  left  is 
simply  Poggendorff's  figure  in  the  horizontal  position,  and 
the  illusion  is  of  the  usual  character.  In  the  figure  at  the 
right,  however,  this  illusory  effect  has  not  only  been  neu- 
tralized, but  actually  reversed,  by  the  change  from  the 
single  pair  of  horizontal  parallels  to  the  more  numerous 
vertical  parallels.1 

This  change  Wundt  is  inclined  to  connect  with  the  broad- 
ening of  the  figure  by  the  division  of  the  space  (cf.  Ex.  195) ; 
but  Jastrow  and  Thiery  with  greater  plausibility  refer  it  to 
the  fact  that  the  angles  in  the  right  figure  are  obtuse.  The 
horizontals  are  indicated,  indeed,  by  the  ends  of  the  verticals, 
but  are  inferior  in  effect  to  the  verticals  actually  present. 

h.  The  Divided  Triangle.  The  horizontal  division  line 
of  the  triangle,  though  placed  at  exactly  half  the  height 
of  the  figure,  seems  too  high,  an  illusion  which,  as  Thiery 
points  out,  the  type-founders  have  regarded  in  the  capital 
A.  The  illusion  holds  with  possibly  greater  effect  when  the 
horizontals  are  replaced  by  concentric  arcs.  For  quantita- 
tive results  on  the  division  of  the  triangle,  see  Thiery,  94  f. 


1  These  figures  are  taken,  by  permission,  from  Ladd's  "  Elements  of  Physio- 
logical Psychology  "  (Copyright,  1887,  by  Charles  Scribner's  Sous). 


199]         VISUAL   PERCEPTION  OF  SPACE,    ETC.         251 


i.  Laska's  Figure.  The  sides  of  the  angle  in  this  figure 
are  equal,  but  are  made  to  seem  unequal 
by  the  setting  of  dots  at  unequal  dis- 
tances from  them.  Laska  offers  no  ex- 
planation for  this  illusion.  Thiery  and 
Miiller-Lyer  regard  it  as  a  special  case  of 
the  contrastive  illusions  (Ex.  197,  above). 

j.    Adjacent  Figures.     In  A  the  adja- 
cent curves  cause  a  slight  illusory  curvature  of  opposite 


252       LABORATORY  COURSE  IN  PSYCHOLOGY.     [199 


o 


direction  in  the  parallels  between  them.  This  figure  might 
therefore  be  regarded  as  an  example  of  the  contrastive 
group.  In  B  the  circles  flatten  each  other.  The  inscribed 
ellipse  distorts  the  square  in  the  direction  of  its  long  axis, 
but  the  circle  in  the  direction  of  the  short  axis. 


k.  There  is  a  certain  tendency  to  see  lines  lying  nearly 
in  the  visual  plane,  and  directed  toward  the  eye,  as  short 
vertical  lines  lying  in  planes  nearly  perpendicular  to  the 
visual  plane.  Tn  the  diagram  above,  the  lines  are  all  di- 
rected to  a  point  eight  or  ten  inches  below  the  bottom  of 
the  diagram.  When  the  eye  is  brought  tc  that  point,  the 


200]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         253 

lines  can  be  seen  with  a  little  effort  as  short  lines  nearly 
perpendicular  to  the  surface  of  the  page. 

I.    In  the  parallel  columns  below  is  shown  another  well- 
known  illusion  :  — 

In  these  two  columns  the  type  is          Here  the  lines  are  "leaded  ;  "i.e., 
of  exactly  the  same  size.     On  this       have  greater  space  between  them. 


According  to  Wundt,  this  is  be-  based  on  the  greater  general  white- 
cause  the  eye  passes  over  the  same  ness  in  this  case  and  blackness  in 
number  of  letters  in  a  shorter  tne  other? 

On  a,  Miiller-Lyer,  A;  Wundt,  A,  4te  Aufl.,  II.,  151  f.  ;  Thiery 
603  f.  On  6,  Delboeuf,  B  and  C;  Wundt,  A,  4te  Aufl.,  II.,  146  f.  ; 
Thiery,  108  f.  On  c,  Delboeuf,  C;  Thiery,  110  ff.  On  <Z,  Miiller- 
Lyer,  A  and  1?  ;  Laska  ;  Lipps,  A.  On  e,  Lipps,  ./I.  On/,  Bradley'  s 
Pseudoptics.  On  y,  Wundt,  A,  4te  Aufl.,  II.,  149;  Jastrow,  ^.  ; 
Thiery,  369  f.  On  h,  Thiery,  94  ff.  ;  Bowditch.  On  i,  Laska;  Thiery, 
84  f.  On  j,  Wundt,  A,  4te  Aufl.,  II.,  150  f  .  ;  Lipps,  J5,  253,  296,  ff.  On 
fc,  Christine  Ladd  Franklin.  On  f,  Wundt,  A,  4te  Aufl.,  II.,  150. 


200.  The  Geometrical  Illusions  with  Unmoved  Eyes. 
Several  of  the  illusions  considered  above  are  weakened 
when  eye-movements  are  excluded.  This  may  be  done  by 
fixation  of  the  eyes,  somewhat  better  by  getting  the  figures 
as  after-images,  and  most  satisfactorily  of  all  by  instan- 
taneous illumination.  Try  the  effect  of  steady  fixation  on 
the  figures  of  Ex.  191.  The  after-image  method  may  be 
tried  on  the  Zollner  figure  (first  figure  under  Ex.  191), 
which  can  be  made  to  give  a  strong  after-image  as  it 
stands,  and  on  any  of  the  other  figures  by  cutting  them 
as  narrow  slits  in  cardboard,  and  then  viewing  them 
against  a  bright  background.  The  method  of  instantane- 
ous illumination  may  be  tried  on  any  of  the  diagrams 
with  the  dark  box  and  photographic  shutter. 

The  fact  that  many  of  these  illusions  are  still  present  in 
a  certain  degree  when  movements  of  the  eyes  are  excluded. 


254       LABORATORY  COURSE  IN  PSYCHOLOGY.      [201 

does  not  necessarily  demonstrate  that  any  part  of  them  is 
of  non-motor  origin.  As  has  already  been  shown  in  Exs. 
172  and  173,  the  experiences  of  the  eye  in  motion  are  re- 
tained, arid  applied  to  its  perceptions  when  at  rest. 

Helmholtz,  A,  G.  709  ff.;  Fr.  725  ff.  (566  ff.);  Wundt,  A,  4te 
Aufl.,  II.,  139;  Thiery,  328  ff. 

EQUIVOCAL  FIGURES. 

In  the  Geometrical  Illusions  the  immediate  cause  of  the 
illusory  perception  seems  to  lie  in  some  peculiarity  of  the 
figures  themselves.  In  the  Equivocal  Figures,  on  the  con- 
trary, the  varying  perceptions  or  interpretations  seem  to 
depend  more  immediately  on  varying  central  or  appercep- 
tive  conditions. 

201.  Plane  Figures.  In  A  and  B  the  black  and  white 
figures  are  precisely  alike,  except  in  position,  and  either 


may  be  taken  as  background  for  the  other.  With  the 
change  of  background  a  change  of  mental"  attitude  is  in- 
volved, depending  in  part  on  this  change,  and  in  part  on 


202]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         255 

the  new  axis  of  symmetry  in  the  subordinate  figures  — 
horizontal  in  A,  and  vertical  in  B,  if  the  ground  is  black ; 
exactly  reversed  if  it  is  white.  Something  similar  happens 
in  (7,  which  may  seem  a  star  made  up  of  interlacing  lines, 


A 


V 


o  o  o  o  o 

o  o  o  o  o 

o  o  o  o  o 

o  o  o  o  o 

o  o  o  o  o 


two  superposed  triangles,  or  a  hexagon  with  six  little  tri- 
angles adjacent.  In  Z>  the  twenty-five  circles  of  the  square 
may  be  grouped  among  themselves  in  many  ways  :  a  single 
square  of  circles ;  five  vertical  or  horizontal  lines ;  two  con- 
centric squares  and  a  central  circle ;  an  equal-armed  cross 
filled  out  with  four  squares  of  four  circles  each,  etc.  A 
little  self-observation  will  probably  show  that  the  change 
of  mental  attitude  leads  at  once  to  a  change  of  eye-move- 
ments, often  merely  incipient,  by  which  the  new  patterns 
are  followed  out. 

Mach,  A,  44  ff.,  87;  von  Bezold,  253;  James,  L,  442  f. 

202.  Diagrams  with  Equivocal  Perspective.  In  the  ac- 
companying figures  it  is  the  interpretation  of  the  space 
relations  of  the  parts  of  the  figure  that  changes.  E  repre- 
sents a  half-open  book,  and  may  be  seen  both  concave  and 


256       LABORATORY  COURSE  IN  PSYCHOLOGY.     [202 

convex,  the  former  probably  being  generally  seen  first. 
F  is  a  glass  tumbler  seen  from  the  top  or  from  the  bottom  ; 
occasionally  also  it  may  appear  bent,  so  that  both  top  and 
bottom  are  turned  toward  the  observer.  In  G  the  curved 
lines  are  subject  to  interpretation  as  concave  at  the  right 
and  convex  at  the  left,  and  vice  versa.  H  is  a  triangular 
pyramid,  of  which  the  longer  side  is  either  nearest  the 
observer  or  farthest  from  him.  It  also  has  two  other 
interpretations;  namely,  as  a  quadrangular  pyramid  looked 
at  from  its  apex,  or  a  hollow  quadrangular  pyramid  seen 
from  below :  the  diagonals  of  the  figure  then  appear  bent 
towards  the  paper  on  either  side  of  the  apex.  /  is  the 
figure  known  as  "Necker's  Cube."  Notice  the  change  in 
the  position  of  the  diagonal  as  the  cube  takes  one  position 
or  the  other.  Also  that  the  farther  side  is  always  a  little 
too  large.  The  inexactness  of  perspective  from  which  the 
latter  comes  (both  squares  are  of  the  same  size)  favors 
a  double  interpretation  of  the  figure.  When  the  perspec- 
tive is  correct  the  inversion  is  more  difficult.  (For  beha- 
vior of  this  figure  by  flash  illumination,  see  Aubert,  A,  618.) 
/is  a  set  of  perspective  cubes,  which  appear  three  in  the 
lower  row,  two  in  the  middle,  and  one  on  top  ;  or  two  in  the 
lower  row,  three  in  the  middle,  and  two  on  top.  This 
figure  is  evidently  a  reduplication  of  Necker's  Cube.  K 
represents  a  pair  of  intersecting  planes, 'with  the  line  of 
intersection  nearly  perpendicular  to  the  paper,  or  nearly 
para'llel  to  it.  L  is  "  Schroder's  Stair  Figure."  It  gene- 
rally appears  first  as  the  upper  side  of  a  flight  of  steps ; 
with  some  effort,  however,  it  may  be  seen  as  the  under 
side  of  such  a  flight,  or  the  overhanging  portion  of  a  wall. 
Wundt  states  that  if  the  eye  follows  the  oblique  lines  of 
the  figure  in  the  direction  b  a,  the  first  form  is  apt  to  be 
brought  in ;  if  in  the  direction  a  b,  the  second.  The 
figure  is  evidently  a  reduplication  of  E.  M  represents 


. 


OF 


202]  VISUAL   PERCEPTION  OF  SPACE,   ETC.         257 


G  H 


BBBBBBBB 


258      LABORATORY  COURSE  IN  PSYCHOLOGY.      [202 

part  of  a  narrow  carved  frieze.  The  little  figures  that 
compose  it  appear  depressed  or  elevated.  With  some  diffi- 
culty a  part  may  be  held  as  depressed  while  the  rest  are 
elevated ;  but  the  result  is  unsteady,  probably  because  we 
are  less  accustomed  to  a  mixture  of  figures  in  such  decora- 
tions than  a  repetition  of  the  same  figure.  N  is  similar  to 
M,  but  introduces  light  and  shade.  Changes  in  the  posi- 
tion of  the  figure  with  reference  to  the  source  of  illumina- 
tion generally  involve  a  change  from  convexity  to  concavity, 
or  vice  versa.  Cf.  also  the  stereoscopic  figures  given  below, 
many  of  which  are  capable  of  double  interpretation  when 
viewed  monocularly. 

All  these  perspective  figures  have,  of  course,  an  inter- 
mediate interpretation  as  plane  figures,  though  this  is 
sometimes  hard  to  hold  after  experimenting  with  the  tri- 
dimensional  interpretations.  Some  of  the  changes  of  form 
are  at  first  a  little  difficult  for  some  observers,  but  once 
gotten  are  easier  to  get  again.  Turning  the  diagram  up- 
side down  is  sometimes  helpful.  Loeb  reports  that  moving 
it  slowly  to  and  from  the  eye  causes  the  figure  to  change 
back  and  forth,  and  Mach  finds  the  changes  brought  about 
by  slow  vertical  movements.  The  Schroder  figure  is  caused 
to  change  by  vividly  conceiving  the  plane  a  as  nearer  than 
b.  It  is  likely  that  these  methods  bring  about  changes  in 
accommodation,  or  in  the  way  in  which  the  eyes  follow  out 
the  figure.  How  far  these  peripheral  changes  would  be 
effective  by  themselves,  or  how  far  apperceptive  changes 
without  peripheral  expression  would  be  effective,  cannot 
now  be  said. 

Notice  that  in  all  the  figures  changes  from  one  interpre- 
tation to  the  other  are  invariably  accompanied  by  other 
changes  of  greater  or  less  extent  in  the  position  and  dimen- 
sions of  the  lines,  angles,  and  surfaces  of  the  figures.  In  E, 
for  example,  when  the  figure  is  convex  (the  middle  line 


203]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         259 

nearer  the  observer  than  the  rest),  all  the  vertical  lines 
incline,  if  at  all,  toward  the  observer ;  when  the  figure  is 
concave,  they  incline  backward,  and  are  longer.  The  rela- 
tive length  of  the  sloping  lines  at  the  top  and  bottom  may 
also  seem  to  change  slightly.  It  is  interesting  to  notice, 
with  Mach,  that  of  the  infinity  of  possible  figures  which 
could  give  these  diagrams  as  geometrical  projections,  only 
a  very  few  extremely  definite  ones  appear  in  perception. 

Helmholtz,  A,  G.  770  ff. ;  Fr.  795  ff.  (626  ff.) ;  Wundt,  A,  4te  Aufl., 
II.,  199  f. ;  Mach,  A,  94  ff.,  B,  405  ff.,  C;  James,  II.,  253  ff. ;  Lange  ; 
Loeb;  Hoppe,  A  and  B  ;  Brewster;  Sully,  95  ff.;  Beaunis,  II.,  569  ; 
Tkiery,  318  ff.,  77,  79  ;  Wheatstone,  A,  381  f. 

203.  Equivocal  Figures  of  Three  Dimensions.  An  in- 
version similar  to  that  observed  in  Ex.  202  can  be  seen 
with  real  objects  when  conditions  are  favorable.  A  simple 
experiment  may  be  made  with  a  visiting-card  bent  in  the 
middle  so  as  to  enclose  an  angle  of  about  120°,  which  gives 
a  figure  resembling  E  above.  Set  the  card  with  the  fold 
vertical  on  a  table,  where  the  light  will  fall  parallel  to  one 
side,  thus  obviating  the  cross  shadows  in  part,  and  look  at 
it  from  a  distance  of  a  couple  of  yards  with  a  single  eye. 
The  card,  like  E,  may  be  seen  either  concave  or  convex. 
Notice  in  this  case,-  as  in  Ex.  202,  the  change  of  dimension 
and  position  that  takes  place  when  the  figure  is  changed 
from  convex  to  concave.  Notice,  also,  that  when  the  card 
is  seen  in  its  illusory  form  (convex  when  it  is  really  con- 
cave, or  vice  versa),  the  shadowed  parts  seem  a  deeper  gray, 
and  the  illuminated  parts  a  brighter  white,  than  when  the 
whole  is  seen  correctly.  The  writer  finds  the  experiment  a 
little  easier  when  the  card  is  on  a  rather  low  table  and  he 
observes  standing,  the  card  then  having  the  top  of  the  table 
as  a  uniform  background. 

Very  fine  effects  are  given  by  casts  of  objects  in  shallow 


260       LABORATORY  COURSE  IN  PSYCHOLOGY.     [204 

relief,  either  in  intaglio  or  cameo  form.  In  these  cases  the 
nature  of  the  object  represented  is  said  to  be  important, 
letters,  numerals,  and  geometrical  figures  turning  easily 
either  way,  but  natural  objects,  human  and  animal  forms, 
and  especially  faces,  turning  easily  from  concave  to  convex, 
but  with  difficulty,  if  at  all,  from  convex  to  concave.  Com- 
pare, for  example,  the  ease  of  seeing  the  concave  mask  in 
Ex.  184  a  in  convex  form  with  the  difficulty  of  getting  a 
convex  mask  to  appear  concave. 

Most  of  the  authors  enumerated  under  Ex.  202  discuss  these  cases 
also. 

BINOCULAR  PERCEPTION  OF  SPACE. 

The  perception  of  relief,  or  the  third  dimension  of  ob- 
jects, is  considerably  improved  by  vision  with  two  eyes. 
This  depends,  as  will  be  shown  in  experiments  below,  upon 
the  difference  in  the  aspects  seen  by  the  two  eyes,  and  this, 
in  turn,  on  the  separation  of  the  eyes.  It  is  clear  that 
the  difference  in  aspect  will  be  important  for  near  objects, 
and  unimportant  for  remote  ones.  For  distances  greater 
than  250  metres  it  is,  by  calculation,  practically  nothing, 
and  for  distances  considerably  less  than  that  must  be  of 
little  influence. 

204.  The  Interocular  Distance.  To  determine  this  dis- 
tance, select  a  distant  fixation  point,  e.g.,  on  the  horizon, 
fix  the  eyes  upon  it,  and  hold  at  arm's  length  before  the 
face  a  pair  of  sharp-pointed  dividers  opened  an  inch  and 
a  half  at  the  tips.  These  will  appear  slightly  blurred  and 
double,  like  a  pair  of  V's  side  by  side,  —V  V.  Still  main- 
taining the  distant  fixation,  gradually  spread  the  dividers 
till  the  inner  points  of  the  double  V  just  touch,  and  the 
whole  becomes  a  W.  Record  the  separation  of  the  points, 
and  repeat  the  measurement,  beginning  this  time  with  the 


205]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         261 

tips  separated  three  inches,  and  gradually  reducing  the  dis- 
tance till  the  W  is  just  formed.  Try  an  equal  number  of 
times  each  way,  and  average  the  results.  If  the  fixation 
has  been  carefully  maintained,  the  separation  obtained  is 
the  interocular  distance.  Le  Conte  recommends  the  use  of 
slightly  convex  spectacles  in  order  to  obviate  the  blurring 
of  the  images.  Verify  the  determination  by  making  in  a 
card  two  fine  pin-holes  separated  by  the  interocular  dis- 
tance found.  Hold  this  as  close  to  the  eyes  as  possible, 
bringing  one  of  the  holes  before  each  eye.  If  the  determi- 
nation is  correct,  the  two  holes  will  fuse  into  one,  when  the 
eyes  are  again  directed  to  the  distant  fixation  point.  Care 
should  be  taken,  however,  not  to  bend  the  card  while  test- 
ing, as  this  changes  the  separation  of  the  holes.  The  aver- 
age distance  is  given  by  Stevens  as  a  little  under  64  mm. 
Le  Conte,  A,  230. 

205.  The  Binocular  Field  of  Vision  and  the  Binocular 
Field  of  Regard.  The  portion  of  visual  space  within 
which  binocular  vision  is  possible  at  any  instant  is  rela- 
tively small,  and  that  within  which  the  binocular  point  of 
regard  may  lie  is  still  smaller. 

a.  The  Binocular  Field  of  Vision.  To  draw  this  field, 
arrange  the  head-rest  of  the  campimeter  in  such  a  position 
that  the  lines  of  sight  may  fall  perpendicularly  in  their 
primary  position  on  the  vertical  plane  of  the  instrument, 
and  the  eyes  may  be  about  10  cms.  distant  from  it.  Fasten 
a  sheet  of  paper  on  the  plane,  and  mark  a  fixation  point 
for  each  eye  immediately  in  front  of  each.  Close  one  eye, 
fix  the  other  upon  its  mark,  and,  keeping  the  eye  and  head 
steady,  dot  the  projected  outlines  of  the  eyebrow  and  nose 
as  seen  in  indirect  vision.  It  will  be  found  convenient  to 
bring  the  point  of  the  pencil  in  from  outside  the  field,  and 
to  fix  points  a  half-inch  or  more  apart.  Repeat  the  pro- 


262       LABORATORY  COURSE  IN  PSYCHOLOGY.     [206 

cess  with  the  conditions  of  the  eyes  reversed.  A  rough 
outline  of  the  field  required  will  thus  be  obtained. 

b.  The  Binocular  Field  of  Regard.  When  the  eyes  are 
used  singly,  each  can  be  brought  to  fixate  practically  all 
the  binocular  field  of  vision ;  but  the  part  within  which  lie 
the  points  that  can  be  fixated  by  both  eyes  at  the  same 
time  —  the  binocular  field  of  regard  —  is  somewhat  smaller. 
This  restriction  is  most  marked  for  distant  points  in  the 
lower  part  of  the  field  because  of  the  tendency  of  the  eyes 
to  converge  as  the  lines  of  regard  are  depressed. 

Fixate  a  distant  spire  or  chimney,  and,  while  maintain- 
ing the  fixation,  throw  the  head  backward  till  the  lines  of 
regard  are  as  nearly  parallel  to  the  face  as  possible.  The 
fixated  object  will  appear  double,  and  closing  one  eye  or 
the  other  will  show  the  images  to  be  such  as  accompany 
convergence  of  the  eyes  upon  a  point  nearer  than  the  object 
fixated.  Cf .  Ex.  208.  A  still  easier  way,  suggested  by 
Hering,  is  to  hold  a  mirror  close  to  the  body,  and  at  such 
an  angle  as  to  give  an  image  of  the  distant  object  nearly 
vertically  below  the  eyes.  The  attempt  to  fixate  has  the 
same  result  as  before.  It  is  important  to  steady  the  head 
to  prevent  its  bending  forward  so  as  to  give  the  eyes  an 
easier  position. 

Bering,  A,  442  ff.;  Aubert,  A,  609,  663  f.;  Helmholtz,  A,  G.  642; 
Fr.  627  (484);  Kirschmann,  458. 

206.  Indistinguishability  of  the  Combined  Fields.  It  is 
often  nearly  or  quite  impossible  for  an  observer  with  nor- 
mal eyes  to  tell  which  eye  is  affected  when  a  change  is 
made  in  one  of  the  superposed  fields.  Try  with  two  tubes 
of  equal  bore,  looking  through  one  with  each  eye  toward 
the  sky  or  other  uniform  background.  Adjust  the  tubes  so 
that  the  circles  of  light  combine  into  a  single  binocular 
image.  When  they  are  exactly  combined,  have  an  assist- 


207]         VISUAL   PERCEPTION   OF  SPACE,   ETC.         263 

ant  thrust  a  pencil-point  before  one  of  them.  It  will  be 
impossible  without  closing  one  eye  to  tell  which  field  has 
been  entered.  If  the  eyes  are  not  alike,  if  they  differ,  for 
example,  in  focal  adjustment,  or  in  degree  or  direction 
of  astigmatism,  means  for  discrimination  will  be  present. 
Helmholtz  finds  in  some  cases  a  more  or  less  unconscious 
differentiation  of  the  fields  when  the  needs  and  habits  of 
vision  favor  it,  and  a  number  of  the  subjects  tested,  in  a 
different  way,  by  Schon  were  able  to  tell  which  eye  was 
illuminated.  Cf.  also  Ex.  218,  and  footnote  appended  to  it. 

Rogers,  C ;  Helmholtz,  A,  G.  893  f •  5  Fr.  938  f .  (743  f . ) ;  Schon,  B, 
83  f.,  C,  61  ff. 

207.  Binocular  Direction.  Each  eye  receives  its  impres- 
sions for  itself  and  from  its  own  station ;  but  in  ordinary 
vision,  at  least  for  points  near  the  point  of  regard,  the  im- 
pressions of  both  eyes  are  united,  and  objects  are  perceived 
as  if  seen  by  a  single  eye  midway  between  the  other  two 
—  the  Cyclopean  Eye  of  Hering.  It  is  from  this  median 
eye  that  binocular  direction  is  taken.1  A  line  drawn  from 
the  centre  of  clearest  vision  in  this  imaginary  eye  to  the 
point  fixated  is  the  Binocular  Line  of  Regard. 

Make  a  pin-hole  in  the  middle  of  a  large  sheet  of  paper. 
Hold  the  paper  as  close  as  possible  to  the  eyes,  and,  be- 
ginning with  the  pin-hole  at  the  extreme  right  of  the  visual 
field,  move  the  paper  slowly  across  the  face.  The  pin-hole 
will  seem  to  come  up  to  the  median  plane,  and  to  pass 
away  at  the  extreme  left,  to  be  succeeded  after  an  in- 
stant by  a  second  pin-hole  that  repeats  the  same  course. 
The  impressions  of  the  eyes  singly  are  referred  one  after 


1  Schon  gives  experiments  to  show  that  objects  at  the  right  and  left  of  the 
point  of  regard  take  their  direction  from  the  eye  on  their  own  side  of  the  head. 
In  the  light  of  the  experiments  of  Loeb  (B),  however,  the  matter  seems  to  me 
still  uncertain.  Cf.  Ex.  180. 


264       LABORATORY  COURSE  IN  PSYCHOLOGY.     [208 

the  other  to  the  cyclopean  eye.  For  other  simple  experi- 
ments on  this  point  see  Helmholtz,  Hering,  and  Wundt  at 
the  places  cited  below. 

Rogers,  A,  325  ff.,  C  ;  Helmholtz,  A,  G.  756  f.,  894  f.;  Fr.  777  f., 
939  f.  (611  f.,  744  f.);  Bering,  A,  386  if.,  540,  B,  28  ff.,  39  ff. ; 
Wundt,  A,  4te  Aufl.,  II.,  175  ;  Le  Conte,  A,  223  ff. 

208.  Single  and  Double  Images,  a.  Hold  up  two  ringers, 
one  about  a  foot,  the  other  about  two  feet,  from  the  eyes. 
Fixate  the  nearer  finger;  it  will  be  seen  single,  and  the 
farther  one  will  appear  double.  Fixate  the  farther  one? 
and  it  will  be  seen  single,  while  the  nearer  appears  double. 
Fixate  the  nearer  finger  again,  and  close  one  eye.  The 
image  of  the  farther  finger  on  the  same  side  as  the  closed 
eye  will  disappear.  Fixate  the  farther  finger,  and  repeat 
the  experiment.  The  image  of  the  nearer  finger  on  the 
side  opposite  to  the  eye  closed  now  disappears. 

The  double  images  in  the  first  case  are  called  Homony- 
mous,  or  Uncrossed  Images  ;  in  the  second,  Heteronymous, 
or  Crossed  Images.  The  hoinonymous  images  in  this  case 
belong  to  the  nasal  halves  of  the  retinas,  and  the  heterony- 
mous  to  the  temporal  halves,  as  can  easily  be  seen  from 
diagram  A,  in  which  L  and  R  represent  the  left  and  right 
eyes  respectively,  b  represents  the  fixation  point,  a  a  point 
more  remote,  and  c  a  nearer  point.  The  accented  letters 
"show  the  place  on  the  retinas  on  which  the  images  of  the 
corresponding  unaccented  letters  lie. 

This  particular  retinal  distribution  is  not  a  necessary 
characteristic,  however ;  for  images  may  be  hoinonymous 
or  heteronymous/and  yet  belong  to  the  nasal  half  of  one 
eye  and  the  temporal  half  of  the  other,  as  shown  for  the 
points  a  and  c  in  diagram  B  (after  Hering).  Roughly 
speaking,  any  point  inside  a  circle  passed  through  the  point 
of  regard  and  the  optical  centres  of  the  two  eyes  will  be 


208]         VISUAL  PERCEPTION   OF  SPACE,   ETC.         265 


seen  in  heteronymous  images,  and  any  point  outside  in 
homonymous  images.  The  nature  of  this  circle  is  consid- 
ered more  fully  in  Ex.  210  and  in  Appendix  II. 

b.  Double  images,  when  both  lie  at  one  side  of  the  point 
of  regard,  are  not  equally  well  perceived.  That  belonging 
to  the  eye  on  the  same  side  being  more  distinct,  and  often 
taken  as  real,  while  the  other  seems  illusory. 


Fix  the  eyes  on  a  distant  object,  and  hold  vertically  a 
little  to  the  right  of  th«  right  line  of  regard,  and  about  six 
inches  from  the  eye,  a  strip  of  paper  a  centimeter  wide  and 
six  or  eight  long,  arranging  it  so  that  both  images  shall  be 
projected  against  a  uniform  background.  The  image  near- 
est the  point  of  regard  will  be  seen  without  difficulty ;  the 
other  will  be  hardly  discernible,  and  will  often  disappear 
entirely  in  rivalry  with  the  part  of  the  background  seen  by 
the  corresponding  part  of  the  right  eye.  In  this  case  the 


266       LABORATORY  COURSE  IN  PSYCHOLOGY.     [209 

images  are  heteronymous,  and  the  strong  image  belongs  to 
the  nasal  half  of  the  right  eye. 

Repeat  the  experiment,  this  time  fixating  a  pencil-point 
held  about  six  inches  from  the  eyes,  and  bring  the  paper 
slip  up  15-20°  to  the  right,  and  six  or  eight  inches  far- 
ther away  than  the  pencil.  In  this  case  it  is  the  image 
nearer  the  point  of  regard  that  is  faint  or  succumbs.  The 
stronger  image  belongs,  as  before,  to  the  nasal  half  of  the 
right  eye. 

Repeat  either  of  the  experiments  just  made,  and  observe 
which  of  the  images  it  is  that  seems  to  be  real  and  held  in 
the  fingers.  A  slightly  increased  pressure  of  the  fingers 
will  probably  increase  the  definiteness  of  the  sensation  in 
question. 

In  general  it  is  the  nasal  images  that  dominate ;  and 
this,  too,  it  is  said,  whether  the  images  are  separate  as  in 
these  experiments,  or  combined  to  a  single  image  indirectly 
seen.  See  Ex.  218,  footnote.  For  other  conditions  influ- 
encing the  perception  of  double  images,  see  Ex.  220. 

Helmholtz,  A,  G.  841  ff.;  Fr.  877  ff.  (695  ff.);  Wundt,  A,  4te 
Aufl.,  II.,  178;  Hering,  A,  397,  424  f.;  Le  Conte,  A,  92  ff.;  Schon, 
A,  B,  and  C. 

209.  Corresponding  Points.  The  phenomena  of  single 
and  double  vision  raise  the  question  of  the  relative  location 
of  the  retinal  points  in  function  in  the  two  cases.  The 
somewhat  full  terminology  of  Wundt  will  assist  in  making 
these  matters  clear.  He  distinguishes  five  sorts  of  points 
(4te  Aufl.,  II.,  173  f.) :  (1)  Identical  Points  are  points  that 
would  exactly  coincide  if  the  retinas  were  superposed ; 
they  might  be  called  geometrical  or  anatomical  correspond- 
ing points.  (2)  Corresponding  Points  are  points  whose 
stimulation  normally  gives  rise  to  single  vision.  (3)  As- 
sociate Points  (Deckpunkte)  are  those  whose  stimulation 


209]          VISUAL  PERCEPTION  OF  SPACE,  ETC.        267 

in  a  given  case  gives  rise  to  single  vision.  (4)  Disparate 
Points  are  non-identical  points ;  and  (5)  Dissociate  Points 
(Doppelpunkte)  are  points  whose  stimulation  in  a  given 
case  gives  rise  to  double  images.  The  dissociate  points 
thus  stand  over  against  the  associate  points  in  the  same 
way  as  the  disparate  over  against  the  identical.  To  make 
the  list  of  terms  complete,  a  set  of  Non-  Corresponding 
Points  might  also  be  distinguished.  Both  the  correspond- 
ing points  and  the  associate  points  are  in  most  cases 
practically  coincident  with  the  identical  points,  and  the 
dissociate  with  the  disparate. 

Corresponding  points  should  not  be  regarded  as  anatomi- 
cally fixed,  but  rather  as  physiological  points  with  a  cer- 
tain range  of  co-operation.  Panum's  view  of  them  as 
retinal  "  sensory  circles,"  the  central  points  of  which  when 
simultaneously  -stimulated  must  give  rise  to  single  vision, 
and  the  peripheral  zones  of  which  may  under  appropriate 
circumstances  do  the  same,  is  in  accord  with  the  facts. 
Where  exactness  of  location  is  important  the  experiment 
must  be  so  arranged  as  to  reduce  the  range  of  co-operation 
as  much  as  possible.  Cf..  Ex.  220. 

a.  Corresponding  Points  are  in  general  similarly  situ- 
ated in  the  two  eyes.  A  rough  demonstration  may  be 
made  as  follows:  Prepare  two  exactly  like  figures  sepa- 
rated from  centre  to  centre  by  the  interocular  distance, 
drawing  one  figure  in  red,  the  other  in  black,  to  hinder 
fusion,  e.g.,  a  set  of  concentric  circles  overlaid  by  a  rec- 
tangular cross.  Combine  these  figures  binocularly  with 
the  haploscope,  or  in  any  other  convenient  way,  fixating 
the  centres  of  the  figures,  and  observe  that  they  coincide 
throughout,  or  with  only  slight  deviations. 

For  accurate  methods  of  determining  the  location  of  cor- 
responding points,  see  Helmholtz,  A,  G.  844  ff.  j  Fr.  880  ff., 
(698  ff.)  ;  Bering,  A,  355  ff. 


268       LABORATORY  COURSE  IN  PSYCHOLOGY.     [209 

b.  Slight  Deviations  from  exact  similarity  in  the  ar- 
rangement of  the  corresponding  points  are  found  in  many 
eyes.  The  most  important  of  these  is  that  affecting  the 
position  of  the  meridian  of  the  apparent  vertical,  which  not 
only  varies  from  subject  to  subject,  but  is  liable  to  a  cer- 
tain variation  in  the  same  subject  within  short  intervals. 

Arrange  the  haploscope  for  parallel  vision,  and  fasten 
upon  the  diagram  board  two  circles  of  cardboard  in  such 
a  way  that  they  may  be  rotated  about  their  centres.  The 
diameters  should  not  exceed  the  interocular  distance,  and 


their  centres  should  be  separated  horizontally  by  exactly 
'that  distance.  The  lines  on  their  surface  should  be  ar- 
ranged as  in  the  cut  above. 

Adjust  one  of  the  disks  so  that  its  lines  are  vertical ; 
then  combine  the  central  lines  of  both  disks  binocularly, 
and  adjust  the  second  disk  till  the  outer  lines  in  the  com- 
bined image  seem  exactly  parallel,  taking  pains  that  the 
plane  of  vision  shall  be  approximately  in  its  primary  posi- 
tion. When  the  position  of  the  disks  is  examined,  it  will 
generally  be  found  that  the  lines  converge  slightly  below. 
Try,  also,  when  the  lines  of  one  disk  are  set  horizontal  and 


210]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         269 

those  of  the  other  are  adjusted  at  right  angles  in  the  com- 
bined image. 

c.  The  Impressions  upon  Corresponding  Points  are  gen- 
erally inseparable.  Stare  fixedly  with  both  eyes  at  a  small 
gas  flame  or  other  bright  object  till  a  strong  after-image 
has  been  secured.  Then  holding  a  page  of  print  before 
the  face,  converge  the  lines  of  regard  upon  a  point  before 
or  behind  it,  or  with  the  finger-tip  gently  push  one  of 
the  eyes  out  of  its  normal  position.  The  images  of  the 
print  will  double,  but  the  after-image  will  remain  single. 
The  after-images  in  the  two  eyes  rest,  of  course,  on  corre- 
sponding points,  and  their  retinal  position  is  unaffected  by 
the  movements  of  the  eyes,  while  that  of  all  other  objects 
is  changed.1 

On  «,  Helmholtz,  A,  G.  844  ff.;  Fr.  880  ff.  (698  ff.);  Hering,  A, 
355  ff.;  Aubert,  A,  605  ff.  On  6,  Helmholtz,  A,  G.  687  f.,  850  ff.; 
Fr.  700  f.,  889  ff.  (546  f.,  703  ff.);  Hering,  A,  358,  368  f.;  Wundt,  A, 
4te  Aufl.,  II.,  140  ff. ;  Aubert,  A,  608.  On  c,  Hering,  A,  433  ;  Schon, 
B,  51  ff. 

210.  The  Horopter.  The  last  experiment  but  one  shows 
that  the  fixation  point  is  seen  single.  Besides  this  there 
are  certain  other  parts  of  the  visual  field  that  may  be 
seen  single  at  the  same  time.  The  sum  of  all  these  parts 
is  called  the  Horopter.  The  horopter  varies  with  the  fixa- 
tion point,  because  the  positions  of  the  eyes  are  different. 
See  experiments  on  eye-movements,  p.  119  ff.  An  exact 
determination  of  its  form  in  any  case  is  extremely  difficult, 

1  Whether  the  sensations  of  corresponding  points  are  ever  separable  is  still 
in  doubt.  Wheatstone  and  others  have  held  that  under  proper  conditions  they 
might  be  separated  and  give  rise  to  double  vision,  just  as  those  of  non-corre- 
sponding points  may  be  united  and  give  rise  to  single  vision.  Hering  and  others 
have  held  the  opposite,  and  devised  experiments  to  show  that  "  Wheatstone's 
Experiment "  is  a  misleading  one.  In  support  of  at  least  a  certain  sort  of 
double  vision  with  corresponding  points,  see  Wheatstone,  A,  384  f . ;  Helmholtz, 
A,  G.  885  ff.;  Fr.  930  ff.  (736  ff.) ;  Wundt,  A,  4te  Aufl.,  II.,  194  ff.  Against  it,  see 
Hering,  A,  434  ff. ;  Aubert,  A,  G09 ;  Schon,  B,  56  ff .,  75  f . 


270       LABORATORY  COURSE  IN  PSYCHOLOGY.     [210 

but  one  or  two  rough  tests  of  special  cases  may  be  made 
without  much  trouble. 

a.  Fasten  two  small  bits  of  cork  on  the  points  of  a  pair 
of  compasses  opened  two  or  three  inches.    Keep  one  steady, 
and  use  it  as  a  fixation  point,  moving  the  other  about  it  in 
circles  in  the  three  principal   planes   of   the  visual  field 
(the  median  plane;   the  plane  of  vision,   i.e.,   the   plane 
passed  through  the  lines  of  regard ;  and  a  vertical  plane 
passed   through   the   fixation   point   perpendicular  to  the 
other  two).     Notice  the  position  of  the  moving  point  when 
double  images  of  it  can  no  longer  be  distinguished.     While 
it  will  not  be  easy  to  fix  these  places  definitely,  certain 
regions  can  be  made  out  within  which  the  double  images 
cannot  be  seen  separately.     One  of  these  will  be  a  broad 
band  extending  above  and  below  the  fixation  point,  and 
another  at   the  right  and  left  of   that   point.     In   these 
regions  lies  the  horopter. 

b.  It  is  easy  to  determine  with  a  certain  accuracy  the 
position  of  the  part  of  the  horopter  above  and  below  the 
fixation  point.     Hang  a  small  weight  by  a  black  thread 
from  a  bar   of   the   window.     Select  a    spot    behind   the 
thread  on  the  glass  at  such  a  height  that  fixation  of  it 
will  bring  the  eye  approximately  into  the  primary  plane. 

•Fixate  the  point  from  a  distance  of  six  or  eight  inches,  and 
observe  that  the  double  images  of  the  thread  are  not 
parallel,  but  form  a  narrow  V  opening  upward.1  Draw  the 
lower  end  of  the  thread  away  from  the  window,  still  main- 
taining the  fixation,  until  the  images  are  parallel.  A 
straight  line  through  the  fixation  point  parallel  to  these 
images  is  the  portion  of  the  horopter  sought. 

c.  A  somewhat  more  definite  notion  of  the  part  of  the 


1  The  threads  may  also  appear  slightly  curved,  with  the  concavity  toward 
the  fixation  point  for  reasons  noticed  in  Ex.  172. 


210]         VISUAL  PERCEPTION  OF  SPACE,  ETC.     .   271 

horopter  at  the  right  and  left  of  the  fixation  point  can  be 
gotten  by  the  following  experiment.  Hold  the  frame  of 
parallel  threads  prepared  for  Ex.  196  c  perpendicular  to 
the  plane  of  vision  in  its  primary  position,  and  six  inches 
or  less  from  the  face  ;  fixate  the  middle  thread,  and  ob- 
serve that  the  threads  seem  to  lie  in  a  cylindrical  surface 
slightly  convex  toward  the  face.  The  apparent  curvature 
is  due  to  the  fact  that  -the  retinal  images  of  the  side 
threads  rest  on  points  which  are  disparate,  though  near 
enough  together  to  serve  for  single  vision. 

If  the  threads,  when  actually  lying  in  a  plane,  appear  to 
lie  in  a  convex  surface,  it  is  clear  that  a  set  of  threads 
lying  in  a  surface  equally  concave ;  i.e.,  when  so  arranged 
as  to  give  retinal  images  resting  on  exactly  corresponding 
points,  would  appear  to  lie  in  a  plane.1  This  fact  Hering 
uses  in  support  of  his  principle  that  points  lying  in  the 
horopter  are  actually  seen  in  the  Surface  of  Single  Vision.2 

As  the  frame  is  again  removed  the  threads  fall  back 
into  a  plane.  This  effect  might,  however,  depend  on  the 
presence  of  the  frame.  To  avoid  this  difficulty,  the  ex- 
periment may  be  made  with  long  threads  viewed  through 
a  tube  so  as  to  cut  off  vision  of  their  ends.  When  thus 
viewed  from  a  distance,  the  threads  seem  to  lie  in  a  plane 

1  The  theoretical  curve  in  which  the  concave  surface  would  cut  the  plane  of 
vision  is  a  portion  of  a  circle  passing  through  the  fixation  point  and  the  optical 
centres  of  the  eyes,  the  "  Circle  of  Muller." 

2  The  Surface  of  Single  Vision,  Hering's  Kernflache,  is  a  transverse  plane,  or 
slightly  concave  surface,  passing  through  the  apparent  point  of  regard  (Kern- 
punkt).     In  it  appear  all  objects  whose  retinal  images  rest  on  exactly  corre- 
sponding points,  or  on  points  that  are  only  vertically  disparate.    Behind  it  is  the 
region  of  homonymous  double  images,  and  in  front  of  it  that  of  heteronymous 
double  images.    Before  or  behind  it  appear  also  all  objects  whose  retinal  images 
rest  upon  disparate  points,  even  when  the  disparateness  is  insufficient  to  cause 
perceptible  double  images.    It  might  be  denned  briefly  as  the  surface  in  which 
the  empirical  horopter  seems  to  lie.    For  usual  positions  of  the  eyes  it  stands 
symmetrical  to  the  median  plane  and  perpendicular  to  the  plane  of  vision,  or 
but  slightly  inclined  away  at  the  top.    With  extreme  positions  of  the  eyes,  how- 
ever, its  position  is  altered.    (Hering  A,  401, 413  ff .) 


272       LABORATORY  COURSE  IN  PSYCHOLOGY.      [211 

(Helmholtz),  or  even  a  concave  surface  (Hillebrand).     For 
quantitative  results,  see  Helmholtz. 

For  other  simple  methods  of  determining  the  horopter 
empirically,  see  Schon,  B,  and  Christine  Ladd  Franklin. 
For  a  geometrical  consideration  of  these  and  other  cases 
see  Appendix  II. 

Helmholtz,  A,  G.  801  ff.,  860ff. ;  Fr.  828  ff.,  901  ff.  (654  ff.,  713  ff. ) ; 
Hering,  A,  375  ff.,  401;  Wundt,  4te  Aufl.,  II.,  189  ff.;  Le  Conte,  A, 
192  fe.,  C,  105  ;  Hillebrand,  A. 

211.  Location  of  Single  and  Double  Images.  The  direc- 
tion in  which  these  images  appear  has  already  been  con- 
sidered in  Ex.  207  ;  in  the  fixing  of  their  apparent  distance 
any  or  all  of  the  criteria  of  visual  distance  may  co-operate. 

a.  Single  Images  at  the  Point  of  Regard.  In  the  ab- 
sence of  other  criteria  single  images  are  located  at  the  dis- 
tance of  the  apparent  point  of  regard.  Use  the  haploscope 
adjusted  for  crossed  vision,  and  any  simple  diagram  in 
which  the  two  pictures  are  exactly  alike  on  the  two  sides. 
The  combined  image  looks  smaller  than  the  original  figures, 
and  hangs  in  the  opening  of  the  transverse  screen.  In- 
crease and  decrease  of  the  distance  of  the  diagram  is 
accompanied  by  similar  movements  of  the  combined  image 
~which  lies  at  the  crossing-point  of  the  lines  of  vision ;  i.e., 
at  the  apparent  point  of  regard. 

Remove  the  diagram  from  the  haploscope,  and  combine 
the  figures  with  free  eyes  and  crossed  lines  of  sight,  or 
use  the  haploscope  after  removal  of  the  transverse  screen. 
Notice  that  all  the  figures  are  now  of  nearly  or  quite  the 
same  size,  though  smaller  than  the  original  pair,  and  lie  in 
the  same  plane.  The  distance  of  the  plane  is  somewhat 
uncertain,  but  seems  greater  than  that  of  the  crossing-point 
of  the  lines  of  regard.  The  knowledge  of  the  actual  dis- 
tance of  the  diagram  by  monocular  and  other  criteria  is 


211]         VISUAL  PERCEPTION  OF  SPACE,  ETC.         273 

partly  preponderant  over  the  datum  from  convergence. 
The  effect  of  the  latter  is  not  wholly  lost,  however,  and 
shows  itself  in  the  decreased  size  of  the  circles.  Cf.  Ex. 
217  a. 

A  similar  experiment  can  also  be  made  by  combining 
the  figures  of  a  uniformly  figured  surface ;  e.g.,  a  papered 
wall  or  an  unpainted  brick  one.  As  successive  figures  are 
combined  by  greater  and  greater  convergence,  a  phantom 
wall  approaches ;  but  location  at  the  crossing-point  of  the 
lines  of  regard  seems  rather  clearer  to  the  writer  when 
the  convergence  is  being  allowed  to  fall  back  stage  by 
stage  than  when  it  is  being  increased.  For  a  special  study 
of  other  phenomena  attending  the  combination  of  such 
figured  surfaces,  see  Le  Conte,  C. 

Combinations  with  parallel  lines  of  sight  ought  on  this 
principle  to  give  combined  images  located  at  an  infinite 
distance,  or,  more  exactly,  at  the  limit  of  distance  for  which 
the  binocular  criterion  is  commonly  useful ;  but  the  effect 
is  hardly  to  be  secured  with  ordinary  diagrams.  The  trans- 
parent one  recommended  in  Ex.  212  b  succeeds  partially, 
when  given  a  not  too  rapid  motion  to  and  from  the  face 
against  a  background  of  sky.  It  is  not  hard,  however,  to 
secure  location  beyond  the  actual  plane  of  the  figures  com- 
bined. Hold  up  an  open-meshed  cane-seated  chair,  six  or 
eight  inches  before  the  face,  and  combine  the  octagons  of 
the  mesh  with  eyes  converged  on  a  more  distant  point. 

For  location  when  the  lines  of  sight  are  divergent,  see 
Ex.  219  a.  For  the  case  of  single  images  showing  binocu- 
lar relief,  see  Ex.  212. 

b.  Double  Images.  The  single  members  of  the  double 
image  pair  are  subject  to  location  according  to  the  usual 
monocular  criteria,  and  to  certain  binocular  criteria  as  well. 
Cf.  Ex.  218  a.  Their  location  as  regards  distance,  espe- 
cially when  seen  with  unmoved  eyes,  is  sometimes  extremely 


274       LABORATORY  COURSE  IN  PSYCHOLOGY.     [211 


uncertain,  and  may  even  be  changed  voluntarily.  The  ten- 
dency under  these  conditions  is  to  locate  them  at  about  the 
distance  of  the  apparent  point  of  regard.  Try  with  a 
couple  of  threads  or  wires  (not  held  in  the  hands)  before 
a  uniform  background,  and  so  arranged  that  the  points 
of  support  cannot  be  seen.  For  careful  experiments  see 
Schon. 


Their  direction  and  approximate  distance,  other  things 
being  equal,  may  be  plotted  schematically  according  to  the 
following  rule  based  on  the  principle  of  binocular  direction 
examined  in  Ex.  207 :  In  order  to  get  the  position  of  any 
pair  of  double  images,  draw  diagrams  showing  the  posi- 
tion of  the  object  giving  rise  to  them  with  reference  to  the 
eyes  singly,  and  then  combine  the  two  by  making  their 


212]        VISUAL   PERCEPTION  OF  SPACE,    ETC.         275 

lines  of  regard  coincide.  The  diagram  opposite,  in  which 
the  middle  one  of  the  three  little  circles  is  taken  as  the 
point  of  regard,  will  make  the  method  clear.  For  fuller 
treatment,  see  Hering,  J5,  and  Le  Conte,  A. 

Brewster,  A,  90  ff. ;  Helmholtz,  A,  G.  795  ff.,  868  f.,  890,  894  f.; 
Fr.  823  ff.,  909  f.,  935,  940  (649  ff.,  720  f.,  740,  744  f.);  Hering,  A, 
426  ff.,  431  f.,  531  ff.,  B,  43,  167  ff.;  Aubert,  A,  613  ff.;  Wundt,  A, 
4te  Aufl.,  II.,  178,  183;  Le  Conte,  A,  112  ff.,  213  ff.;  Schon,  B,  100 
f.,  C,  50  ff.;  Hyslop,  B. 

212.  Binocular  Perception  of  Eelief.  This  rests  on  the 
perceptive  union  of  the  slightly  different  images  received 
by  the  two  eyes.  Those  parts  of  the  images  which  lie  at 
any  instant  on  pairs  of  corresponding  points  are  located  in 
the  Surface  of  Single  Vision;  those  that  lie  on  disparate 
points,  though  they  may  be  seen  single,  are  located  before 
or  behind  that  surface  —  before  it,  if  the  disparateness  is 
such  that  when  carried  to  a  greater  extent  it  would  give 
heteronymous  double  images  ;  behind  it,  if  such  as  to  give 
homonymous  images  :  or,  to  state  the  same  thing  in  terms 
of  eye-movements,  those  parts  of  the  object  will  seem 
nearer  for  the  exact  fixation  of  which  an  increased  conver- 
gence would  be  required,  and  those  parts  will  seem  more 
remote  for  which  a  less  convergence  would  be  required. 

a.  Try  with  diagrams  like  those  on  the  following  page, 
combined  binocularly  with  the  stereoscope,  haploscope,  or 
with  free  eyes.  In  the  first,  the  right  and  left  figures  are 
exactly  alike.  In  the  others  they  are  different  in  such  a 
way  as  to  make  the  figure  convex  in  the  second  diagram 
and  concave  in  the  third  —  supposing  the  combination  to 
be  made  with  lines  of  regard  parallel,  or  nearly  so  —  or 
concave  in  the  second  and  convex  in  the  third,  if  the  lines 
of  regard  cross  before  the  diagram. 

The  effect  of  identical  pictures  may  easily  be  seen  in  the 
Wheatstone  stereoscope  by  turning  one  of  the  figures  up- 


276      LABORATORY  COURSE  IN  PSYCHOLOGY.     [212 


side-down,  provided  the  figures  are  drawn  in  the  middle  of 
the  cards.  Nos.  1  and  24  of  Martius-Matzdorff's  diagrams 
show  the  same  thing  excellently  in  the  ordinary  stereoscope. 


b.  Combination  with  Free  Eyes.  The  experimenter  will 
find  it  convenient  to  secure  the  ability  to  combine  such 
binocular  diagrams  with  free  eyes.  The  first  attempts  at 
combination  with  parallel  lines  of  sight  should  be  made 


212]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         277 

with  diagrams  in  which  the  corresponding  parts  are  at 
most  no  farther  apart  than  the  centres  of  the  eyes ;  in 
ordinary  stereoscopic  pictures  the  distance  is  too  great. 
The  easiest  way  in  which  to  begin  is  to  use  a  diagram  on 
glass,  which  may  be  easily  prepared  as  follows.1  On  a 
suitably  sized  piece  of  glass  stick  two  small  paper  dots, 
with  their  centres  separated  by  the  interocular  distance. 
About  these,  but  with  their  centres  a  couple  of  millimeters 
nearer  together,  stick  two  narrow  paper  rings ;  and  outside 
these,  again,  two  larger  rings,  with  their  centres  coinciding 
with  those  of  the  dots.2  The  resulting  diagram  will  be 
like  the  second  one  above.  Hold  the  glass  plate  at  arm's 
length,  and  looking  through  it,  fixate  some  very  distant 
object.  The  two  figures  will  instantly  combine,  and  the 
smaller  ring  will  take  its  place  before  the  other.  In  trying 
with  ordinary  diagrams,  bring  the  card  against  the  fore- 
head, allow  the  eyes  to  take  an  unconstrained  position, 
then  move  the  diagram  slowly  away.  Combination  with 
parallel  lines  of  regard  is  favored  by  holding  the  diagram 
in  such  a  position  that  the  eyes  must  turn  upward  to  see 
it,  the  parallel  position  of  the  lines  of  regard  being  habit- 
ually associated  with  elevation  of  them.  Four  figures  are 
apt  to  be  seen  at  first,  the  middle  two  of  which  can,  with 
care,  be  brought  together  and  combined. 

To  combine  the  figures  by  crossing  the  eyes,  hold  the 
diagram  at  a  convenient  distance,  and  bring  between  it  and 
the  face,  in  the  median  plane,  a  pencil-point  or  other  small 
object.  "Fixate  the  pencil-point,  and  notice  the  images,  as 
in  the  previous  case.  If  just  the  right  distance  has  been 
hit,  three  images  only  will  be  seen ;  if  fotfr,  move  the  pen- 

1  The  writer  is  indebted  for  the  suggestion  of  this  experiment  to  the  ex- 
planatory text  accompanying  Martius-Matzdorff' s  diagrams. 

2  For  these  dots  and  rings  nothing  is  more  convenient  than  those  prepared 
for  kindergarten  use,  —  "  Mrs.  Hailmann's  dots,"  and  "  gummed  paper  rings." 


278       LABORATORY  COURSE  IN  PSYCHOLOGY.      [213 

cil  backward  and  forward  till  the  middle  two  have  been 
brought  together. 

One  difficulty  in  using  the  free  eyes  comes  from  the  ha- 
bitual association  of  accommodation  for  distant  vision  with 
parallelism  of  the  lines  of  regard,  and  accommodation  for 
near  vision  with  convergence  ;  but  sufficient  practice  will 
train  the  eyes  to  a  more  or  less  complete  dissociation  of 
these  functions. 

For  literature,  see  any  general  account  of  binocular  vision. 

213.  Increase  of  the  Binocular  Criterion  ;  the  Telestereo- 
scope.  The  telestereoscope  is  an  instrument  for  increas- 
ing the  difference  in  the  images  of  real  objects  received 
by  the  two  eyes,  and  so  the  binocular  factor  in  the  percep- 
tion of  their  relief.  Its  principle  will  be  easily  understood 
from  the  plan  opposite,  in  which  the  heavy  lines  represent 
mirrors  and  the  light  lines  show  the  course  of  the  rays  of 
light. 

Objects  reflected  in  the  large  mirrors  are  again  reflected 
in  the  small  mirrors,  and  so  reach  the  eyes  at  L  and  R. 
The  right  eye  thus  sees  objects  as  it  would  if  stationed  at 
r,  and  the  left  as  if  at  I.  For  practical  purposes  the  inter- 
ocular  distance  has  been  made  equal  to  I  r. 

Arrange  the  Wheatstone  stereoscope  for  use  as  a  tele- 
stereoscope  by  turning  the  diagram  holders  around  and 
moving  them  out  to  the  ends  of  the  bed.  Place  it  before 
an  open  window  overlooking  a  landscape  containing  a  good 
deal  of  detail,  or  even  at  one  end  of  a  large  room  —  though 
in  this  case  it  may  be  advisable  to  reduce  the  separation 
of  the  mirrors  somewhat.  Adjust  the  instrument  till  a 
clear  and  easily  combined  view  of  the  landscape  is  secured. 
The  result  will  be  a  decided  increase  in  the  binocular  relief 
of  the  objects  seen.  The  effect  is  often  that  of  a  small 
model,  in  which,  of  course,  for  the  normal  eyes  there  would 


213]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         279 

be  the  same  ability  to  see  more  than  usual  of  both  sides 
of  the  houses  and  trees,  combined  with  the  same  small  reti- 
nal images.1 

Considerable  trouble  may  be  met,  if  the  instrument  is 
roughly  made,  in  getting  the  images  given  by  the  large 
mirrors  to  stand  at  exactly  the  same  height,  but  care  in 


A 


6 


adjustment  will  bring  them  where  they  belong.  The  final 
position  should  be  such  as  not  to  cause  undue  straining 
of  the  eyes.  As  in  so  many  other  binocular  experiments, 


1  The  difference  in  the  binocular  aspects  is  not  the  only  factor,  however  ;  for 
mere  increase  of  convergence,  without  increase  of  the  interocular  base-line, 
causes  a  very  similar  effect,  as  may  he  seen  by  examining  the  landscape  with  a 
10-20°  prism  held  before  one  eye  (sharp  edge  toward  the  nose),  while  the  other  eye 
remains  free. 


280       LABORATORY   COURSE  IN  PSYCHOLOGY.      [214 

the  effect  will  probably  become  more  marked  as  the  eyes 
are  moved  about  from  object  to  object  and  the  distances 
studied. 

Helmholtz,  A,  G.  793  f.,  831;   Fr.  821  f.,  861  (647  f.,  681). 

214.  Keversal  of  the  Binocular  Criterion ;  the  Pseudo- 
scope.  The  effect  of  the  pseudoscope  upon  an  object  seen 
through  it  is  equivalent  to  an  interchange  of  the  images 
received  by  the  eyes,  so  that  the  one  seen  by  the  right  eye 
is  like  that  usually  seen  by  the  left,  and  vice  versa.  The 
result,  when  proper  conditions  are  observed,  is  a  reversal 
or  conversion  of  the  binocular  relief  of  the  object.  The  in- 
strument consists  of  two  total-reflection  prisms,  set  with 
their  reflecting  sides  vertical,  and  inclining  a  little  toward 
the  median  plane. 


The  right  eye  sees  the  right  side  of  the  object  reflected 
in  the  right  prism,  and  thus  reversed.  The  left  eye  sees 
the  left  side  of  the  object  similarly  reflected  and  reversed. 
Points  in  the  object  which  normally  require  increased  con- 
vergence now  require  less,  and  vice  versa.  In  use,  the  in- 
strument should  be  adjusted  by  varying  the  separation  of 
the  prisms  and  their  inclination  to  the  median  plane  till 
the  images  fuse  easily,  and  with  about  the  degree  of  con- 
vergence required  for  normal  vision  of  the  object. 

The   pseudoscopic    effect    seems   quite   capricious,  some 


215]         VISUAL   PERCEPTION   OF  SPACE,    ETC.         281 

objects  instantly  appearing  in  changed  relief,  others  refus- 
ing to  change.  It  is  most  certain  with  simple  geometrical 
models  of  wire  and  other  forms  that  are  equally  familiar 
in  both  kinds  of  relief  (e.g.,  the  medallions  of  Ex.  203), 
and  in  which  the  monocular  criteria  are  weak  or  wanting. 
In  other  cases  the  reversal  is  difficult,  or  only  partial.  Try 
with  simple  forms  of  wire,  and  then  with  more  complicated 
ones,  such  as  boxes,  bottles,  or  vases,  and  finally  with  the 
human  face  —  a  small  bust,  or  the  face  of  an  assistant.  In 
all  cases  care  must  be  taken  to  avoid  the  interference  of 
monocular  criteria. 

All  the  more  important  pseudoscopic  phenomena  can  be 
gotten  with  stereoscopic  diagrams  when  those  intended 
for  the  right  eye  are  seen  by  the  left.  The  ordinary  geo- 
metrical forms  turn  instantly  when  they  are  combined 
alternately  by  straight  and  crossed  vision,  either  with  free 
eyes  or  the  haploscope.  The  results  are  partial  and  doubt- 
ful when  more  complicated  figures  are  tried,  especially 
if  there  is  shading  and  the  mathematical  perspective  is 
strong.  With  stereoscopic  photographs  failures  are  fre- 
quent. Try  with  any  convenient  set  of  stereoscopic  pho- 
tographs. 

Wheatstone,  B,  10  ff. ;  Brewster,  A,  208  ff. ;  Helmholtz,  A,  G. 
791  ff.;  Fr.  819  ff.  (646  ff.);  Aubert,  A,  625;  Le  Conte,  A,  139  f.; 
Stevens,  A,  447. 

215.  Judgments  of  Depth  with  Two  Eyes.  Absolute 
judgments  of  depth  are  difficult  to  experiment  upon  be- 
cause of  the  difficulty  of  excluding  relative  judgments  of 
various  kinds.1  When  these  are  excluded  the  judgments 
are  extremely  uncertain,  as  will  be  shown  in  Ex.  217.  For 
estimates  of  the  distances  of  unknown  objects  under  such 
conditions,  see  Wundt,  B,  and  Eouse. 

1  Cf.  the  remarks  made  upon  absolute  judgments  of  the  position  of  the  lines 
of  regard,  p.  19U. 


282       LABOEATOEY  COURSE  IN  PSYCHOLOGY.     [216 

Judgments  of  Relative  Depth  are  easy  to  experiment 
upon,  and  show  a  surprising  accuracy.  In  the  following 
diagram  the  spacing  of  the  letters  is  not  strikingly  differ- 
ent when  observed  in  the  ordinary  way.  When,  however, 
the  two  figures  are  combined  binocularly,  considerable  dif- 
ferences of  level  are  to  be  observed. 


IN 

THE    SAME 
PLANE? 


IN 

THE    SAME 
PLANE? 


The  same  applies  to  figures  of  any  kind  which  show 
slight  differences  in  the  relative  position  of  their  parts. 
Indeed,  the  differences  necessary  to  give  relief  are  so 
slight  that  it  is  extremely  difficult  to  draw  two  figures, 
e.g.,  squares  or  hexagons,  that  will  lie  flat  when  binocularly 
combined.  Nos.  34  and  35  of  Martius-Matzdorff's  series 
are  excellent  for  demonstrating  this  effect.  For  quantita- 
tive measurements  of  the  accuracy  of  binocular  judgments 
of  relative  distance,  see  Helmholtz,  A,  G.  790 ;  Fr.  817  f . 
(644  f.)  ;  and  Wundt,  A,  4te  Aufl.,  II.,  135,  and  B. 

Helmholtz,  A,  G.  788  ff.,  795  ff.;  Fr.  814  ff.,  823  if.  (642  ff.,  649 
ff.);Hering,  ^4,413  ff.,  551. 

216.    Binocular  vs.  Monocular  Localization. 

a.  At  a  little  distance  before  a  uniform  background 
stretch  a  thread  vertically  with  a  bit  of  lead.  Observe  the 
thread  from  a  distance  of  8  or  10  inches,  holding  a  piece  of 
cardboard  of  nearly  that  length  in  the  median  plane  in  such 
a  way  as  to  shut  off  from  the  right  eye  all  objects  to  the 


216]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         283 

left  of  the  thread,  and  from  the  left  all  objects  to  the 
right.  Fixate  the  thread  steadily  for  a  short  time,  and 
then  attempt  to  touch  it  with  a  pencil  or  teasing-needle 
brought  up  on  one  side  perpendicular  to  the  median  plane. 
The  pencil  will  of  course  be  seen  by  one  eye  only  until  it 
is  near  the  thread.  Steadily  maintaining  the  fixation  of 
the  thread,  bring  the  pencil  up  within  two  or  three  inches, 
and  then  with  a  rapid  movement  attempt  to  touch  the 
thread.  The  pencil  will  be  found  to  pass  behind  it.  This 
is  the  form  of  the  experiment  given  by  Helmholtz,  who 
says,  further,  that  the  error  is  small  if  the  head  is  brought 
into  position  with  the  eyes  closed,  and  the  touching  is  done 
immediately  after  opening  the  eyes,  but  that  the  error  in- 
creases with  long  fixation,  perhaps  on  account  of  fatigue 
of  the  eye-muscles. 

In  repeating  the  experiment,  the  writer  has  found  the 
illusion  exactly  reversed  by  a  very  slight  change  in  condi- 
tions. If  the  fixation  of  the  thread  is  not  continuously 
maintained,  but  the  pencil  itself  is  directly  fixated  and 
adjusted  to  the  apparent  distance  of  the  thread  (monocu- 
larly  of  course,  except  so  far  as  the  other  eye  converges 
consensually),  the  touch  will  fall  short  instead  of  passing 
beyond. 

b.  Differences  between  monocular  and  binocular  location 
can  also  be  shown  with  diagrams.  Draw  a  couple  of  heavy 
circles  with  centres  four  or  five  inches  apart.  Combine 
them  with  crossed  vision,  and  hold  a  pen  at  the  intersec- 
tion of  the  lines  of  regard.  If  the  pen  is  now  moved 
rapidly  to  and  fro  through  the  point  of  intersection,  it  will 
be  seen  now  nearer  and  now  farther  than  the  central  circle, 
as  might  be  expected ;  but  if  it  is  moved  nearer  the  dia- 
gram and  kept  there,  or  if  it  is  at  first  brought  up  from 
that  side  of  the  point  of  intersection,  the  central  circle  is 
apt  to  lie  in  or  near  the  plane  of  the  diagram,  or  at  least 


284       LABORATORY  COURSE  IN  PSYCHOLOGY.     [216 

beyond  the  pen,  notwithstanding  that  the  double  images  of 
the  latter  are  homonymous.  The  nearer  location  of  the 
pen  probably  depends  on  the  knowledge  of  the  distance  of 
the  hand  that  holds  it,  the  greater  degree  of  accommoda- 
tion required  to  see  it  clearly  (accommodation  is  for  the 
true  distance  of  the  diagrams  if  their  lines  are  seen  sharp), 
and  the  overlapping  of  the  circle  by  the  images  of  the  pen, 
when  that  occurs.  These  unite  to  give  the  pen  its  nearer 
and  approximately  correct  location.  It  is  hardly  necessary 
to  point  out  the  small  importance  which  such  experiments 
allow  to  the  convergence  of  the  lines  of  regard  as  a  visual 
criterion.  Cf.  also  Ex.  217  a. 

Somewhat  similar  differences  of  location  are  to  be  noticed 
when  the  intersecting  circles  of  the  following  diagram  are 
united  binocularly.  The  result  of  union  is  three  over- 
lapping circles.  The  side  circles  may  appear  either  before 
or  behind  the  central  one,  or  all  may  seem  to  lie  in  sepa- 
rate planes. 


c.  Several  competent  experimenters  report  that  objects, 
especially  those  at  a  distance,  look  smaller  when  viewed 
with  a  single  eye.  Try  the  experiment,  cutting  off  the 
view  of  one  eye  or  the  other  with  a  bit  of  cardboard.1 

1  The  writer  has  not  had  uniform  success  with  this  experiment,  perhaps 
from  the  neglect  of  some  condition  not  specified  by  those  who  have  reported  it, 


217]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         285 

d.  It  is  by  no  means  easy  to  secure  either  binocular 
or  monocular  vision  by  itself.  The  following  experiment 
shows  the  co-operation  of  the  second  eye  under  circum- 
stances which  might  at  first  seem  monocular.  Provide  a 
picture  showing  a  considerable  stretch  of  distance,  cut  off 
the  vision  of  one  eye  by  holding  a  card  an  inch  or  so  before 
it,  fixate  a  point  in  the  extreme  foreground,  e.g.,  the  edge 
of  the  picture,  and  remove  the  card.  The  change  in  ap- 
pearance will  be  slight.  Cover  the  eye  again,  select  a  point 
in  the  extreme  background,  and  get  a  clear  perception  of 
its  remoteness  by  comparing  its  distance  with  that  of  ob- 
jects in  the  foreground.  Finally  fixate  the  point  selected, 
and  remove  the  card.  Double  images  of  the  point  will  be 
seen  for  an  instant,  showing  that  the  eyes  have  assumed 
a  degree  of  convergence  suitable  for  objects  more  distant 
than  the  plane  of  the  picture. 

Helmholtz,  A,  G.  796;  Fr.  824  (650);  Hyslop,  A  and  B;  James, 
II.,  143;  Aubert,  A,  620. 

217.  Changes  in  Convergence  and  in  the  Size  of  the 
Retinal  Images. 

a.  Changes  in  Convergence,  with  Constant  Retinal 
Images.  This  matter  has  already  been  somewhat  con- 
sidered in  Ex.  211  a,  but  the  method  here  described  has 
certain  advantages.  Adjust  the  Wheatstone  stereoscope 
so  that  combination  of  the  diagrams  takes  place  without 
strain  upon  the  eyes.  Then  move  both  arms  of  the  instru- 
ment at  the  same  time  slowly  backward  from  the  observer, 
continuing  the  combination.  An  increasing  convergence 
will  be  required,  and  the  apparent  size  of  the  combined 

or  perhaps  from  some  difference  in  his  eyes.  When  he  succeeds,  the  decrease 
does  not  occur  instantly,  but  a  second  or  two  after  the  interposition  of  the  card. 
The  decrease  of  size,  when  it  occurs,  seems  to  be  connected  with  a  nearer  loca- 
tion of  the  object ;  but  at  times  there  is  a  reverse  location,  and  objects  seem 
farther  away. 


286       LABOEATOEY  COURSE  IN  PSYCHOLOGY.     [217 

image  will  decrease,  though  the  distance  of  the  diagrams 
from  the  eyes,  and  consequently  the  size  of  their  retinal 
images,  remains  constant.  Moving  the  arms  back  again 
causes  an  apparent  enlargement  of  the  combined  image. 
The  same  experiment  may  be  tried  roughly  by  cutting 
apart  an  ordinary  binocular  diagram,  holding  the  parts  at 
arm's  length,  combining  them  by  crossed  vision,  and  gradu- 
ally separating  the  pictures  while  still  maintaining  the 
combination.  The  combined  image  will  grow  distinctly 
smaller  as  the  figures  are  separated,  and  enlarge  as  they 
approach  each  other.  Examination  of  the  combined  image 
in  either  case  may  show  that  its  final  situation  is  nearer  or 
farther,  but  during  the  movement  the  change  of  size  is  the 
more  apparent.  The  convergence  criterion  has,  during  the 
movement,  little  influence  on  the  perceived  distance,  which 
is  otherwise  determined ;  but  its  change  is  effective  in  the 
apparent  reduction  in  size.  In  both  cases  accommodation, 
if  the  images  are  kept  clear,  remains  constant ;  and  its 
influence,  if  it  has  any,  will  be  in  favor  of  constancy  of 
apparent  distance. 

v  Retinal  images  of  constant  size  can  also  be  secured  as 
after-images,  and  then  the  degree  of  convergence  of  the 
closed  eyes  can  be  altered  without  any  direct  effect  of 
accommodation.  Try  with  the  after-image  of  a  gas  flame, 
gotten  from  a  distance  of  eight  or  ten  inches.  Conver- 
gence will  often  cause  both  decrease  of  size  and  nearer 
location.  Such  is  the  writer's  experience,  but  Scharwin 
and  Novizki  report  a  different  result  under  circumstances 
apparently  the  same  in  all  essential  particulars. 

b.  Effects  of  Change  in  the  Size  of  the  Retinal  Images 
without  Change  of  Convergence.  Adjust  the  instrument 
as  for  a  above.  Then  slide  both  diagrams  at  the  same  time 
toward  the  mirrors  or  away  from  them.  The  result  will 
be  a  decrease  or  increase  of  the  apparent  distance  of  the 


218]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         287 

combined  image,  though  the  degree  of  convergence  has  re- 
mained unchanged.  The  difference  in  size  of  the  retinal 
image  can  be  recognized  as  such  ;  but  the  change  of  distance 
seems  the  readier  interpretation,  probably  because  of  the 
knowledge  of  the  simultaneous  movement  of  the  diagrams. 
The  state  of  convergence  seems  wholly  ineffective,  and 
the  result  is  exactly  like  that  observed  when  the  experi- 
ment is  made  with  a  single  eye.  In  this  case  accommoda- 
tion changes  with  change  in  the  distance  of  the  pictures. 

The  same  may  be  tried  with  the  haploscope  arranged  for 
parallel  vision,  or  with  the  ordinary  stereoscope  by  simply 
sliding  the  diagram  holder  toward  the  eyes  or  away  from 
them. 

Wheatstone,  B,  2  ff. ;  Helmholtz,  A,  G.  795;  Fr.  823  (649);  Hille- 
brand,  A,  42;  Rogers,  A,  93  ff.;  Stevens,  B,  292  ff.;  Judd. 

218.  Movement  of  the  Eyes  in  the  Perception  of  Belief. 
It  was  supposed  by  some  of  the  earlier  investigators  that 
successive  fixation  of  the  different  parts  of  an  object  was 
necessary  to  the  binocular  perception  of  its  relief,  or,  in 
other  words,  that  movement  of  the  eyes,  by  which  the  parts 
were  successively  seen  single,  was  essential.  Such  move- 
ment is  probably  a  considerable  help,  but  is  by  no  means 
necessary. 

a.  Hering's  Experiment  of  the  Falling  Ball.  Arrange 
.a  pasteboard  tube  wide  enough  to  admit  of  binocular  vision, 
and  about  a  foot  long,  so  that  it  shall  look  toward  a  white 
background  a  couple  of  yards  away.  A  little  distance  from 
the  end  of  the  tube  set  up  a  large  screen  pierced  by  a  nar- 
row horizontal  slit ;  e.g.,  5  mm.  wide  and  150  mm.  long.  A 
yard  or  so  from  this  hang  a  plumb-line,  made  of  a  silk 
thread  and  a  bit  of  lead,  in  such  a  way  that  neither  end 
of  it  can  be  seen  through  the  slit.  Let  the  observer  look 
through  the  tube  and  fixate  the  thread.  Then  let  the 


288       LABORATORY  COURSE  IN  PSYCHOLOGY.      [218 

experimenter  drop  a  small  bullet  or  large  shot  from  a  mod- 
erate height,  a  little  before  or  behind,  and  a  little  to  one 
side  of  the  thread.  As  it  passes  the  level  of  the  slit,  it  will 
be  seen  for  an  instant  by  the  observer ;  and  he  will  readily 
be  able  to  tell  which  has  been  done,  unless  the  distance  of 
the  line  of  fall  has  been  very  nearly  that  of  the  thread.  The 
size  of  the  shot  used  should  be  unknown  to  the  observer, 
and  may  well  be  changed  from  trial  to  trial.  It  should 
also  be  caught  in  the  hand  or  on  a  thick  cloth  at  the  end 
of  its  fall,  to  prevent  judgments  based  on  the  sound  of  its 
striking.  Try,  also,  a  few  times  with  monocular  vision  for 
purposes  of  comparison  and  as  a  check.  The  observer 
should,  in  this  case,  fail  about  as  often  as  he  succeeds, 
while  the  distances  from  the  thread  are  still  such  as  to 
be  almost  always  correctly  judged  with  two  eyes. 

b.  Bring  the  thread  up  within  a  foot  of  the  tube,  and 
increase  the  distance  before  or  behind  it  at  which  the  shot 
is  dropped,  till  double  images  of  the  latter  can  be  detected. 
Notice  that  they  also  are  recognized  as  before  or  behind  the 
thread.     As  it  is  somewhat  difficult  to  see  double  images 
under  these  circumstances,  the  shot  must  be  dropped  as 
nearly  as  possible  in  line  with  the  thread,  so  as  to  bring 
the  images  on  either  side  of  it. 

c.  Instantaneous    Illumination    furnishes   a   method  of 
more  varied   application.      Arrange  the  dark   box  for  in- 
stantaneous illumination.     Prepare  several  stereoscopic  dia- 
grams for  special  use  with  the  box,  drawing  them  in  heavy 
lines  on  opaque  cardboard,  and  making  the   distance  be- 
tween the  symmetrical   points   of   the  paired  figures  not 
greater  than  the  interocular  distance.    Through  the  middle 
points  make  fine  needle-holes.     Put  one  of  the  diagrams  in 
place  on  the  back  wall  of  the  box ;  let  the  observer  bring 
the  two  needle-holes  to  combination  with   either  crossed 
or  uncrossed  vision,  and  illuminate  the  diagram.     If  the 


219]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         289 

light  is  sufficient  and  the  relief  of  the  picture  clear,  the 
relation  of  the  parts  will  be  recognized  instantly.  If  a 
single  illumination  is  not  sufficient,  allow  more,  but  at 
intervals  of  a  few  seconds.1 

These  experiments  may  be  varied  by  instantaneous  illu- 
mination of  real  objects  instead  of  diagrams,  or  still  more 
simply  by  sending  single  sparks  through  a  perspectively 
placed  Geissler  tube,  or  by  observing  the  irregular  course 
of  long  induction  sparks. 

Another  method  of  demonstrating  binocular  perception 
of  relief  without  eye-movements  has  been  used  by  Rogers 
and  other  observers.  It  depends  on  the  combination  of 
after-images  successively  produced  in  the  two  eyes,  but  is 
rather  difficult  of  execution. 

Helmholtz,  A,  G.  889  ff.,  Fr.  934  ff.  (739  ff.);  Hering,  A,  407  f., 
427  ;  Rogers,  B;  Stevens,  B  ;  Le  Conte,  A,  148  ff.  ;  Greeff;  Du  Bois- 
Reymond  ;  Aubert,  A,  617. 

219.  Unusual  Eye-movements  in  Favor  of  Binocular 
Combination.  Few  of  the  possible  movements  of  the  eyes 
are  under  direct  voluntary  control,  but  almost  any  can  be 
brought  about  indirectly  and  in  a  slight  degree  by  making 
them  temporarily  necessary  for  single  vision. 

a.  Divergent  Movements.  Slight  divergent  movements 
may  be  induced  by  bringing  the  arms  of  the  Wheatstone 
stereoscope  gradually  toward  the  observer  beyond  the  point 
giving  parallel  vision,  while  he  maintains  the  combination 
of  the  diagrams. 


*  In  order  that  the  relief  may  be  instantly  and  correctly  perceived,  it  is  obvi- 
ous that  the  impressions  received  by  the  two  eyes  must  never  be  interchanged  ; 
that,  as  Le  Conte  says,  each  eye  should  "  know  its  own  image."  General  com- 
parison of  the  monocular  fields  shows  little  to  prevent  an  interchange  (cf.  Ex. 
206),  but  the  acute  observations  of  Schb'n  have  demonstrated  that  the  sensations 
produced  at  corresponding  points  differ  in  definiteness,  in  intensity,  in  color,  in 
their  persistence  in  rivalry,  and  in  still  other  respects  ;  and  on  these  differences 
probably  rests  the  certainty  of  correct  binocular  interpretation. 


290       LABORATORY  COURSE  IN  PSYCHOLOGY.     [219 

The  same  may  be  secured  with  a  diagram  like  that  given 

in  miniature  in  the  margin,  the  observer  first  combining 

figures  separated  by  his  interocular  distance,  or  less,  and 

then  advancing  slowly,  step  by  step,  to  those  of  greater 

and  greater  separation.     The  centres  of 

^-^        ^— .         each  pair  of  small  circles  are  the  same 

\O/       \Q)        distance    apart   as   the    centres    of    the 

^-.          ^-^        large  circles  in  the  figure  immediately 

(Q)        v-y        below;  when,  therefore,  the  smaller  base 

^-^  ._.         of  the  first  cone  is  seen  single,  the  larger 

(O)         (Q)       base  of   the  second  is  seen  single  also, 

^_^^          ^^        and  so  on.     A  certain  aid  may  perhaps 

(O)         vQ/       ke  Derived  from  the  conception  of  greater 

distance  in  passing  from  the  large  to  the 

(O)          (O)       small  circles,  but  a  diagram  in  which  the 

circles   are  concentric   seems    nearly  or 

X~~N.  X~~^ 

(O)          (O)      quite  as  helpful.    Notice  also  that  diver- 
gence makes  no  difference  in  the  median 
(Q)      forward  location  of  the  combined  image, 
and  little  in  its  apparent  distance. 

b.  Asymmetrical  Movements.  It  is 
easy  to  cause  one  eye  to  turn  upward 
while  the  other  remains  at  rest,  by  giv- 
ing a  corresponding  movement  to  one  of 
the  figures  in  the  Wheatstone  stereo- 
scope or  the  haploscope ;  or  to  cause  one 
eye  to  move  inward  and  upward  while 
the  other  moves  inward  and  downward,  by  turning  an  ordi- 
nary stereoscopic  diagram  in  its  own  plane,  while  combined 
with  free  eyes.  In  all  cases  the  movement  must  be  very 
gradual ;  arid,  if  the  combination  tends  to  break  up,  a  little 
time  must  be  given  for  recombination,  or  a  return  made  to 
an  easier  stage. 

Something  similar  may  be  accomplished  with  a  prism  of 


220]         VISUAL  PERCEPTION  OF  SPACE,   ETC.         291 

small  angle  first  held  before  the  eye,  with  the  sharp  edge 
toward  the  nose,  and  then  turned  slowly  about  the  line  of 
sight. 

c.  Rotation  of  the  Eye  about  the  Line  of  Sight.  This 
may  be  induced  by  gradual  rotation  of  one  or  both  of  the 
figures  of  a  stereoscopic  pair  about  their  centres,  during 
combination.  It  is  well  to  use  a  fairly  complicated  geo- 
metrical figure ;  and  success  must  be  judged  by  the  hori- 
zontals, not  by  the  verticals,  for  the  latter  may  remain 
single  without  rotation  of  the  eye'. 

Helmholtz,  A,  G.  631  if.,  799  f.,  Fr.  615  ff.,  827  f.  (474  ff.,  653  f.) ; 
Bering,  A,  504  ff. ;  Le  Conte,  A,  252  ff. ;  Stevens,  A,  B,  290  ff. 

220.  Conditions  that  Help  and  Hinder  the  Seeing  of 
Double  Images.  One  of  these  conditions  has  already  been 


mentioned  incidentally  in  Ex.  209  a ;  a  few  others  are 
gathered  here.  The  ability  to  distinguish  double  images 
seems  to  differ  considerably  from  person  to  person  ;  and  the 
specifications  here  given  may  for  that  reason  be  unsuited 
to  some  observers,  but  the  alterations  required  will  be 
obvious. 

a.  Like  Diagrams  of  Slightly  Different  Size  may  be 
combined  without  double  images.  Try  with  the  diagram 
above,  in  which  the  right  circle  is  a  millimeter  greater  in 


292       LABORATORY  COURSE  IN  PSYCHOLOGY.      [220 

diameter  than  the  left,  or  with  Martius-Matzdorif's  dia- 
gram No.  13. 

The  experiment  may  also  be  made  by  gradually  moving 
one  of  the  pictures  away  from  the  mirror  (or  toward  it)  in 
the  Wheatstone  stereoscope.  Wheatstone  points  out  that 
the  union  of  images  of  unequal  size  is  normal  in  vision  of 
objects  at  the  extreme  right  and  left  of  the  binocular 
field  of  regard. 

b.  Images  Doubled  Vertically  are  more  easily  distin- 
guished than  those  doubled  horizontally.  Try  with  the 
diagram  below  (after  Wundt).  The  second  circle  from 
the  centre  and  the  outer  one  are  alike  in  the  two  figures, 
the  first  and  third  are  unequal.  The  latter  combine  at  the 
sides,  but  show  double  images  above  and  below. 


c.  Slight  Differences  in  the  Figures  to  be  combined,  if 
not  easily  capable  of  a  spatial  interpretation,  may  hinder 
combination  and  favor  double  images.    Try  with  the  figures 
below. 

Covering  a  portion  of  the  right  line  in  one  of  the  figures 
of  the  first  diagram  has  something  of  the  effect  of  the  dot 
in  the  second  and  the  cross  line  in  the  third. 

d.  Movement  of  the  Eyes,  which  brings  one  part  of  the 
images  after  another  on  corresponding  retinal  points,  tends 
to  obscure  double  images  ;  steady  fixation  tends  to  bring 


220]        VISUAL   PERCEPTION  OF  SPACE,   ETC.         293 


them  out,  especially  after  retinal  rivalry  begins.  Try 
with  the  first  of  the  diagrams  below.  For  this  reason  the 
presence  of  lines  in  the  figures  which  tempt  the  eye  to 
movement  (Wundt's  fixation  lines)  are  often  a  hindrance 
to  the  appearance  of  double  images.  Cf.  a  similar  effect  of 
eye-movements  on  after-images,  Ex.  126. 


e.  If  fixation  is  not  maintained  with  care,  double  images 
may  be  neglected  even  near  the  point  of  regard.  Combine 
the  crosses  in  the  diagram  below,  and  observe  that  the 
vertical  lines  at  the  right  form  a  more  or  less  exactly  con- 
tinuous line  in  the  combined  image.  Observe,  further,  that 
slight  efforts  toward  increased  or  decreased  convergence, 
which  may  even  occur  spontaneously,  cause  these  lines  to 
slide  a  little  with  reference  to  each  other,  while  the  central 
cross  yet  remains  single. 

Helmholtz,  A,  G.  874  ff.,  Fr.  916  ff.  (725  ff.);  Hering,  A,  432  f.; 
Wheatstone,  A,  385  f . ;  Rogers,  A,  (XX.)  331  ff.  (XXI.)  85  ff.,  181; 
Wuudt,  4te  Aufl.,  II.,  392  ff. 


294       LABORATORY  COURSE  IN  PSYCHOLOGY.      [221 

221.  Stereoscopy;  Further  Examples.  The  cases  given 
below  do  not  differ  in  any  essential  particular  from  those 
already  considered  in  Ex.  212.  They  have  a  certain  in- 


terest, however,  in  presenting  the  same  principles  under  a 
variety  of  circumstances. 

a.  Stereoscopy  with  Moving  Figures.  On  a  disk  pre- 
pared for  the  rotation  color-mixer  draw  two  heavy  circles 
arranged  as  in  A  below,  the  large  one  exactly  concentric 
with  the  disk,  the  small  one  eccentric  to  it.1 

Place  the  disk  on  the  spindle  of  the  color-mixer,  facing 
the  observer,  and  ten  or  twelve  feet  away.  Before  his  left 
eye  place  a  total-reflection  prism  with  its  reflecting  surface 
parallel  to  his  median  pla*ne  and  its  right-angle  edge  verti- 

1  The  following  dimensions  will  answer  :  Diameter  of  disk,  12  in. ;  diameter 
of  large  circle,  10  in. :  diameter  of  small  circle,  Gin. ;  distance  of  centre  of  small 
circle  from  centre  of  disk,  half  an  inch  ;  lines  of  circles  about  one-tenth  of  an 
inch  wide. 


221]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         295 

cal  —  one  of  the  prisms  of  the  pseudoscope  answers  every 
purpose.  Set  the  disk  with  the  small  circle  at  its  greatest 
horizontal  excursion  on.  the  right  or  left,  and  have  the 
observer  examine  it,  looking  with  his  left  eye  through 
the  prism,  and  with  his  right  eye  directly  at  the  disk.  If 
binocular  combination  is  impossible,  adjust  the  prism  till 
it  is  secured.  Then  set  the  disk  in  slow  rotation,  and 


the  observer  will  see  the  inner  circle  advance  and  retreat 
through  the  larger  one  with  a  sort  of  piston  motion  that 
is  very  striking.  Associated  with  this  movement  are 
changes  in  the  apparent  size  of  the  moving  circle.  The 
reason  for  the  advance  and  retreat  will  be  clear  when 
it  is  noticed  that  the  disk  and  its  reflection  as  they  turn 
produce  in  succession  stereoscopic  pairs  in  the  form  of  B 
and  C,  with  intermediate  positions  that  serve  well  enough 
to  complete  the  forward  and  backward  movement. 

A  simpler  but  less  striking  experiment  can  be  made  as 
follows :  Make  three  little  plumb  lines  with  as  many  bul- 
lets and  silk  threads,  leaving  the  threads  three  or  four 
feet  long ;  hang  these  about  three  inches  apart  in  one  plane, 


296       LABOEATOEY  COURSE  IN  PSYCHOLOGY.      [221 

before  a  white  background,  at  such  a  height  that  the  bullets 
will  be  about  the  height  of  the  eyes.  Use  the  bullets  as 
stereoscopic  figures,  and  combine  them  by  crossing  the  eyes 
so  as  to  produce  four  images  in  which  the  middle  pair  con- 
sists of  the  image  of  the  actual  middle  one  combined  with 
each  of  the  outside  ones.  Now,  preserving  the  combination, 
set  the  middle  bullet  swinging  a  very  little  in  the  plane  of 
the  threads.  The  result  will  be  an  apparent  swinging 
of  both  middle  images  in  planes  at  a  considerable  angle  to 
the  actual  plane,  and  in  opposite  directions. 

I.  The  Binocular  Stroboscope.  If  a  moving  object  is 
presented  to  the  two  eyes  in  slightly  different  positions, 
its  two  images,  though  not  exactly  synchronous,  may  be 
combined  binocularly,  and  the  object  given  a  correspond- 
ing location.  Try  with  the  binocular  stroboscope.  Place 
the  three  disks  upon  the  spindle  of  the  color-mixer  in  the 
order  A,  B,  (7,  so  as  to  make  a  combination  like  D  in  the 
cut  below,  taking  pains  that  the  slits  in  A  and  B  shall  lie 
in  radii  a  few  degrees  apart,  and  that  the  band  on  C  is 
vertical  when  the  line  mn  which  bisects  the  angle  between 
the  slits  is  horizontal.  Place  the  color-mixer  in  a  good 


light,  and,  facing  it  at  a  distance  of  about  two  feet,  a 
mirror  large  enough  to  show  the  whole  of  disk  C.  Close 
to  disk  A  on  the  side  away  from  the  mirror  place  a  black 
cardboard  screen  pierced  by  a  slit  of  length  sufficient  for 


221]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         297 

the  use  of  both  eyes,  and  not  wider  than  the  inner  end  of 
the  slits  in  B.  When  the  screen  is  in  position  the  slit 
should  lie  radially  to  the  disk  at  the  height  of  its  centre, 
and  allow  the  observer  to  see  the  reflection  of  the  disks  in 
the  mirror.  When  the  disk  is  set  in  rotation  he  sees  the 
band  on  C  not  lying  in  the  plane  of  the  disks,  but  inclined 
to  it.  Neither  rate  nor  direction  of  rotation  makes  any 
difference  in  the  apparent  inclination,  but  change  of  the 
relative  position  of  the  slits  in  A  and  B  will  reduce  the  in- 
clination to  zero,  or  change  its  direction.  The  cause  of  the 
apparent  inclination  will  be  readily  seen  on  examination  of 
the  images  received  by  the  single  eyes. 

The  phenomenon  is  elegantly  demonstrated  with  the 
form  of  apparatus  used  by  Dvorak,  but  the  principle  in- 
volved is  the  same. 

c.  Stereoscopy  by  Difference  in  Color ;  Einthoven's  Ex- 
periment. This  depends  on  the  chromatic  aberration  of 
the  eye  already  noticed  in  Ex.  109  b  and  184  b.  On  a 
background  of  black  velvet  paste  at  2  cm.  intervals  alter- 
nate strips  of  blue  and  red  paper  —  strips  1  cm.  wide  and 
8-10  cm.  long.  Place  the  diagram  in  a  good  light  at  a  dis- 
tance of  three  or  four  meters,  and  look  at  it  with  both  eyes. 
The  different-colored  strips  will  not  appear  to  lie  in  the 
same  plane,  some  observers  seeing  the  red  nearer  than  the 
blue,  others  seeing  the  blue  nearer.  Try  also  monocularly, 
for  comparison ;  the  difference  in  distance  will  be  feeble  or 
wanting. 

The  aberration  depends  chiefly  on  a  slight  eccentricity 
of  the  pupil,  and  the  illusion  may  be  increased  or  reversed 
by  partial  covering  of  the  pupil.  If  the  red  has  seemed 
nearer,  a  covering  of  the  nasal  halves  of  the  pupils  will 
bring  the  red  still  nearer,  and  covering  the  temporal  halves 
will  advance  the  blue.  An  examination  of  the  diagrams 
accompanying  the  explanation  in  Ex.  184  b  in  connection 


298       LABORATORY  COURSE  IN  PSYCHOLOGY.     [221 

with,  the  following  simple  experiment  will  make  clear  the 
origin  of  the  stereoscopic  effect. 

On  the  middle  of  a  strip  of  blue  paper  about  one  cm. 
wide  by  fifteen  cm.  long,  paste  a  strip  of  red  paper  of  the 
same  width  and  five  cm.  long,  so  as  to  make  a  party-colored 
strip  of  three  sections,  the  end  ones  blue,  the  middle  one 
red.  From  this  party-colored  strip  carefully  cut  a  strip 
five  mm.  wide  and  fifteen  cm.  long,  and  mount  it  on  a  black 
velvet  background.  Place  it  in  a  good  light,  and  view  it 
from  a  distance  of  three  or  four  meters  with  a  single  eye 
and  half-covered  pupil.  If  the  right  eye  is  used,  the  cover- 
ing of  the  temporal  half  shifts  the  red  portion  of  the  strip 
to  the  right  5 l  if  the  nasal  half  is  covered,  the  red  is  shifted 
to  the  left.  The  same  will  be  found  true  for  the  left  eye, 
with  the  interchange  of  the  terms  temporal  and  nasal. 
Now,  when  the  nasal  halves  of  both  pupils  have  been 
covered,  the  resultant  images  will  be  somewhat  as  in  the 
accompanying  diagram,  where  the  single  line  stands  for 
blue  and  the  double  line  for  red. 


Left  eye's  image. 


Right  eye's  image. 


It  is  evident  that  a  greater  degree  of  convergence  would 
be  necessary  for  combining  the  images  of  the  red  part  of 
the  strip  than  for  combining  the  blue. 

d.    Anaglyphs.     It  is  not  necessary  that  the  stereoscopic 


3  The  same  of  course  would  be  true  if  both  were  shifted,  the  red  to  the  right, 
the  blue  to  the  left ;  but  as  a  matter  of  convenience  1  have  spoken  as  though  the 
red  alone  were  shifted. 


221]         VISUAL  PERCEPTION   OF  SPACE,    ETC.         299 

figures  should  be  separate,  provided  that  they  are  so  ar- 
ranged that  each  eye  gets  its  own  figure  only.  This  may 
be  accomplished  by  giving  a  distinct  color  to  the  figure  for 
each  eye,  and  then  looking  at  the  combination  through 
glasses  or  gelatine  sheets  of  such  color  as  to  allow  each  eye 
to  see  only  its  own  figure.  Various  means  have  been  used 
for  accomplishing  this  result.  A  simple  demonstration  of 


it,  however,  may  be  made  as  follows :  Cut  a  piece  of  black 
cardboard  of  such  size  that  it  will  just  fit  into  the  window 
frame  over  one  of  the  panes.  In  this  cut  three  narrow  verti- 
cal slits  arranged  like  those  in  A  above — making  the  distance 
between  a  and  b  about  two  inches,  and  that  between  b  and 
G  about  a  quarter  of  an  inch.  The  slits  themselves  may 
be  three  or  four  inches  long  and  an  eighth  of  an  inch  wide. 
On  the  back  of  the  sheet,  covering  the  slit  a,  paste  a  strip 
of  white  writing-paper ;  behind  b,  two  thicknesses  of  red 
gelatine ;  and  behind  c,  one  thickness  of  green  gelatine  and 
one  of  blue.1 


1  This  combination  works  tolerably  with  the  ordinary  gelatine  on  hand  at 
present  in  the  Clark  laboratory.     With  other  kinds  of  gelatine  other  combina- 


300       LABORATORY  COURSE  IN  PSYCHOLOGY.      [221 

Place  the  diagram  in  the  window-frame  at  such  a  height 
that  it  will  have  a  sky  background,  and  look  at  it  through 
gelatine  combinations  like  those  used  in  making  it ;  i.e., 
two  thicknesses  of  red  gelatine  before  one  eye  and  a  thick- 
ness each  of  green  and  blue  before  the  other.  If  the  red 
is  before  the  right  eye  the  combined  lines  at  the  right 
will  seem  before  the  plane  of  the  cardboard ;  if  before  the 
left  eye,  they  will  seem  behind  it.  If  the  effect  is  not 
clear  at  first,  it  may  possibly  be  helped  by  a  little  volun- 
tary increase  and  decrease  of  convergence.  The  familiar 
truncated  cone  would  be  shown  by  a  figure  like  B  above, 
when  properly  supplied  with  colored  gelatine  behind  the 
inner  circles  and  paper  behind  the  outer. 

e.  Stevens's  Figure.  Prepare  for  the  Wheatstone  stereo- 
scope a  pair  of  identical  diagrams,  each  composed  of  three 
or  four  heavy  concentric  circles.  Set  them  in  the  frames, 
and  combine  them  stereoscopically.  The  result  will  be,  of 
course,  a  flat  figure.  Then  turn  the  frames  slowly  about  a 
vertical  axis  in  such  a  way  as  to  make  the  pictures  more 
and  more  nearly  face  the  observer.  The  result  will  be 
a  bulging  forward  of  the  central  circles,  giving  the  whole 
the  appearance  of  an  elliptical  shield  or  watch-glass  seen 
from  the  convex  side.  Turning  the  frames  in  the  other 
direction  produces  a  similar  concave  effect.  The  experi- 
ment can  be  made  with  equal  success  with  a  stereoscope  or 
haploscope  provided  with  frames  that  can  be  turned,  or 
even  with  free  eyes  and  diagrams  held  in  the  hands.  Le 
Conte  shows,  however,  that  the  effect  is  mixed,  being  due 


tions  may  be  necessary.  The  thing  to  be  sought  is  a  combination  that  will  stop 
off  the  red  light  as  fully  as  possible.  Whether  the  combination  is  the  one 
required  can  easily  be  judged  by  looking  through  it  at  a  bit  of  the  red  gelatine. 
If  the  latter  looks  black  or  very  dark  the  combination  will  answer.  Any  other 
pairs  of  complementary  colors  would  of  course  answer  as  well  as  the  red  and 
green  used  here. 


221]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         301 

in  part  to  simple  geometrical  projection,  and  in  part  to  bi- 
nocular combination. 

On  a.  Helmholtz,  A,  G.  838,  Fr.  869  f.  (688.);  Christine  Ladd 
Franklin,  111.  On  b.  Dvorak.  On  c.  Einthoven;  Briicke,  II.,  198  f., 
Brewster,  A,  126  ff.  On  e.  Stevens,  B,  297  ff. ;  Le  Conte,  B,  104. 

VISUAL  PERCEPTION  OF  MOVEMENT. 

In  the  case  of  a  slow-moving  object  like  a  planet,  or 
even  the  hour-hand  of  a  watch,  the  movement  is  clearly 
inferred  from  seeing  the  object  successively  in  different 
places.  In  the  case  of  more  rapidly  moving  objects  the 
movement  seems  to  be  immediately  perceived.  It  is  with 
the  latter  that  this  section  is  to  deal.  The  first  question 
would  naturally  be  that  of  the  rate  at  which  inference 
gives  place  to  immediate  perception.  This  is  by  no  means 
easy  to  determine,  because  the  processes  of  inference  and 
perception  are  extremely  difficult  to  separate ;  they  go  on 
at  the  same  time,  and  are  alike  in  character.  Aubert  and 
others  have  put  the  rate,  under  favorable  conditions,  at 
from  one  to  two  minutes  of  arc  per  second  —  the  eye 
being  regarded  as  the  centre.  It  is  not  difficult  to  find 
objects  in  motion  at  about  that  rate.  The  minute-hand 
of  a  full-sized  watch  (moving,  say,  2  mm.  per  minute  at 
the  tip),  when  viewed  at  a  distance  of  about  nine  inches 
(23  cm.),  gives  the  slower  of  these  rates,  and  other  rates 
may  be  obtained  in  a  similar  way  from  clocks  of  different 
size.1  The  large  role  of  inference  in  these  perceptions 
accounts  in  part  for  the  great  difficulty  of  perceiving  the 
motion  of  a  single  point  of  light  in  a  dark  field. 

When  the  rate  of  movement  is  sufficiently  rapid  to  make 

1  To  the  writer  the  movement  of  the  minute-hand,  even  when  the  watch  is 
brought  closer  to  the  face  than  23  cm.,  seems  irregular  and  more  certainly  per- 
ceptible when  the  hand  is  just  passing  one  of  the  minute-marks,  which  would 
indicate  thsit  for  him,  at  least,  the  rate  is  below  that  of  genuinely  perceived 
motion. 


302      LABORATORY  COURSE  IN  PSYCHOLOGY.      [223 

immediate  perception  an  important  factor,  two  methods 
of  using  the  eyes  can  be  distinguished.  The  eyes  may 
be  fixed  upon  the  moving  object,  and  move  as  it  moves, 
the  retinal  image  of  the  object  thus  remaining  nearly 
stationary,  while  the  images  of  all  other  objects  move  ; 
or  the  eyes  may  be  kept  stationary,  in  which  case  the 
retinal  image  of  the  moving  object  moves,  and  those  of 
other  objects  remain  fixed.  The  methods  do  not  always 
give  the  same  results  in  perception. 

222.  Von  FleischPs  Experiment.     The  rate  of  objects 
in  rather  rapid  motion  seems  considerably  greater  when 
the  eyes  are  at  rest  than  when  the  eyes  follow  —  twice 
as   great,   according   to   von    Fleischl   and   Aubert.     The 
experiment  can  easily  be  made,  when  riding  in  a  street- 
car or  carriage,   by  comparing  the  apparent  rate  of   the 
ground  when  the  eyes  are  fixed  on  one  point  of  it  after 
another  with  the  apparent  rate  when  they  are   fixed  on 
the  step  or  some  part  of  the  framework  of  the  vehicle. 
In  the  laboratory  the   experiment  is  conveniently  made 
with  a  rotating  drum  covered  with  paper  carrying  strongly 
marked  lines  or  bands  transverse  to  the  direction  of  mo- 
tion, before  which  a  wire  or  standard  is  placed  for  fixation, 
or  even  with  a  simple   pendulum  made  of   a  thread  and 
a  bit  of  lead;  as  in  Ex.  224.1 

Von  Fleischl ;  Aubert,  C  ;  Stern. 

223.  Positive  After-images  of  Motion.2     When  the  eyes 
are   suddenly  closed  after  a  brief   fixation  of   a  moving 

1  In  trying  with  the  drum  and  a  rate  of  movement  of  about  5  cm.  per  second, 
the  writer  found  the  increase  of  apparent  rate  fairly  clear  when  changing  from 
eyes  in  motion  to  eyes  at  rest.    Decrease  in  the  apparent  rate  in  changing  in 
the  reverse  order  seemed  a  good  deal  less  certain. 

2  These  after-images  should  logically  have  been  treated  with  the  negative 
after-images  of  motion  in  Ex.  128,  but  at  the  time  that  experiment  was  described 


223]         VISUAL   PERCEPTION  OF  SPACE,   ETC.         303 

object,  it  is  possible  to  observe  for  an  instant  an  apparent 
continuance  of  the  movement  in  its  original  direction.  It 
seems  likely  that  both  methods  of  seeing  motion  (eyes 
moved  and  eyes  at  rest)  furnish  such  images,  the  first 
through  actual  continuance  of  the  movement  of  the  eyes 
after  the  fall  of  the  lids,  and  the  second  through  a  per- 
ceptive inference  based  upon  the  positive  after-images 
present.  In  studying  the  images  it  is  of  course  essential 
to  keep  these  kinds  separate,  and  to  distinguish  both  from 
the  "  primary  memory,"  or  "  memory  after-image  "  (James, 
I.,  643  ff.),  though  in  normal  vision  all  probably  co-operate. 
The  experiments  seem  to  the  writer  by  no  means  easy,  and 
he  gives  them  with  some  hesitation.  The  following  direc- 
tions, though  the  best  that  trial  has  yet  suggested,  may 
easily  be  superseded  as  the  phenomena  receive  fuller  study. 

a.  The  After-images  Following  Observation  with  Moved 
Eyes  can  be  secured  by  closing  the  eyes   suddenly  after 
observing  vehicles  passing  on  the  street.     The  motion  may 
at  times  seem  to  continue  after  the  retinal  after-image  has 
entirely  faded ;  and  this,  with  the  clear  subjective  impres- 
sion of  change  in  the  direction  of-  the  eyes,  seems  to  point 
to  a  continuation  of  their  actual  movement.     The  noise  of 
the  vehicle   may,   and   probably  does,   contribute    to  this 
effect,  and  in  so  far  the  case  is  not  a  pure  one.     Something 
similar    may   be  observed    after    glancing   at  a   swinging 
pendulum.     Here  noise  is  not  a  factor. 

b.  The  After-images  Following  Observation  with  Eyes 
at  Kest  are  best  secured  by  very  brief  regarding  of  a  fairly 
rapid   movement.      The    observation   can    be   made    most 
easily  when  travelling  by  rail,  upon  objects  situated  two 
or  three  rods  from  the  track  on  the  opposite  side  of  the 

the  writer's  attention  had  never  been  called  to  them.  Their  importance  for  the 
general  theory  of  the  visual  perception  of  movement  is  sufficient  justification 
for  their  consideration  here. 


304      LABORATORY  COURSE  IN  PSYCHOLOGY.      [223 

car  from  that  on  which  the  observer  is  seated.  When 
such  objects  are  likely  to  be  passed,  the  observer  should 
select  a  point  on  the  forward  side  of  the  window  frame  as 
a  fixation  point,  and  then  close  his  eyes,  or  turn  them  away 
from  the  window.  When  his  eyes  are  free  from  after- 
images, he  should  open  them,  or  turn  them  again  to  the 
window,  fixate  the  selected  mark  for  perhaps  half  a  second, 
close  his  eyes,  and  notice  instantly  the  apparent  movement 
of  the  objects  seen  in  the  after-image.  In  a  fraction  of  a 
second  this  first  stage  of  the  after-image  is  passed  and  the 
usual  sequence  of  colors  begins  (Ex.  125  d).  In  this,  noth- 
ing of  the  moving  objects  can  be  observed,  but  the  definite- 
ness  of  the  image  of  the  window  will  testify  to  the 
approximate  constancy  of  fixation.  In  the  laboratory  the 
moving  image  may  be  secured  from  a  white  disk  a  foot  or 
so  in  diameter,  carrying  a  number  of  heavy  radial  bands 
like  the  spokes  of  a  wheel.  A  disk  of  this  sort,  when  ro- 
tated rapidly  enough  to  blur  the  outer  ends  of  the  spokes 
slightly,  gives  the  effect  upon  closure  of  the  eyes  or  the 
interposition  of  a  piece  of  black  cardboard. 

This  experiment  seems  to  show  that  the  perception  of 
movement  with  the  eyes  at  rest  is  based  upon  the  per- 
ceptive interpretation  of  the  fading  train  of  positive  after- 
images left  by  the  moving  retinal  image,  and  in  much 
the  same  way  that  the  perception  of  relief  is  based  on  a 
perceptive  interpretation  of  darkened  colors  in  the  shad- 
owed parts  of  objects.1  Otto  Fischer  was  led  to  the  same 
opinion  by  experiments  of  quite  a  different  character ;  see 
his  paper,  pp.  144  f. 

Stern. 

1  A  certain  support  for  this  view  is  also  to  be  found  in  the  methods  sometimes 
used  in  rough  sketches  for  indicating  the  movement  of  flying  cannon-balls  and 
the  like.  It  would  be  interesting  to  see  whether  a  drawing  of  an  object,  followed 
by  a  shading  that  should  fairly  counterfeit  the  after-image  train,  would  give  the 
impression  of  movement  when  seen  by  instantaneous  illumination. 


224]         VISUAL  PERCEPTION  OF  SPACE,  ETC.         305 

224.    Perception  of  Motion  in  Indirect  Vision. 

a.  Movement  may  be  perceived  with  indirect  vision 
when  the  points  marking  its  beginning  and  end  are  too 
near  together  to  be  distinguished  with  certainty  when  at 
rest.  Adjust  the  head-rest  of  the  campimeter  eight  or  ten 
inches  from  the  plane ;  fasten  upon  it  a  strip  of  paper  a 
few  inches  wide  and  a  couple  of  feet  long,  placing  the  strip 
horizontal  with  its  middle  in  the  median  plane.  Near  the 
right  end,  and  just  within  the  field  of  the  left  eye,  place 
four  black  dots,  each  about  an  eighth  of  an  inch  in  diam- 
eter, at  the  corners  of  a  half-inch  square.  Close  the  right 
eye,  and  find  with  the  left  a  fixation  point  at  such  a  dis- 
tance to  the  left  that  the  black  dots  can  no  longer  be 
distinguished.  In  finding  this  point,  the  dots  must  be 
covered  most  of  the  time  with  a  bit  of  paper  like  that 
on  the  plane,  and  the  observation  must  be  made  at  the 
instant  that  they  are  uncovered,  for  retinal  impressions 
in  the  periphery  fade  with  the  greatest  quickness.  Having 
found  the  fixation  point  required,  bring  into  the  field 
radially  from  still  farther  to  the  right  a  narrow  strip  of 
the  same  paper,  carrying  at  its  end  a  dot  like  the  four 
just  mentioned.  Observe  that  it  is  possible  to  perceive 
movements  of  this  dot  which  are  less  in  extent  than  the 
interval  between  the  fixed  dots,  even  when  it  lies  farther 
than  they  from  the  fixation  point.  Measurements  by 
Aubert  and  Stern  showed  the  limen  of  perceptible  motion 
to  be  higher  in  the  periphery  than  the  centre  of  the  field, 
though  not  so  much  higher  as  might  have  been  expected 
from  the  poor  discrimination  of  the  periphery  for  objects 
at  rest.  The  difference  between  the  centre  and  periphery 
of  the  eye  in  perceiving  the  flicker  of  black  and  white 
disks,  mentioned  in  Ex.  145  c,  is  perhaps  a  related  phe- 
nomenon. 

I.    The  statement  is  sometimes  made  that  the  apparent 


306 


LABORATORY  COURSE  IN  PSYCHOLOGY.     [224 


rate  of  moving  objects  seen  indirectly  is  greater  than 
that  of  the  same  objects  seen  directly,  and  certain  experi- 
ments seem  to  give  ground  for  the  statement.  It  is  prob- 
able, however,  that  this  is  an  error,  and  that  an  important 
factor  has  been  omitted  in  the  interpretation  of  the  ex- 
periments; namely,  that  in  direct  vision  the  eye  follows 
the  moving  object,  and  in  indirect  it  does  not.  When  this 
difference  is  avoided,  it  is  hard  to  perceive  a  difference  in 
rate  in  the  two  conditions. 

Hang  a  bit  of  lead  by  a  thread  a  foot  or  less  in  length, 
so  that  it  shall  swing  pendulum-wise  to  and  from  the 
surface  of  a  mirror.  Provide  a  uniform  background  for 
the  pendulum,  and  place  close  to  the  latter  something  to 
serve  as  a  fixation  mark.  Take  such  a  position  that  the 
pendulum  and  its  reflection  shall  be  seen  at  nearly  the 
same  distance,  and  look  at  the  pendulum  as  it  swings. 
T^he  reflected  pendulum  will  seem  to  make  somewhat 
greater  excursions,  and  to  move  a  little  faster  in  making 
them.  Having  observed  this,  compare  the  two  rates  when 
the  eye  is  steadily  held  at  the  fixation  point.  The  quick- 
ening observed  in  Ex.  222  may  appear  in  the  pendulum 
itself,  but  little  if  any  difference  can  be  observed  between 
the  pendulum  and  its  reflection.1  If  a  regularly  rotating 
drum  is  at  hand,  the  experiment  may  be  made  very  con- 
veniently by  covering  the  drum  with  paper  lined  over  with 
heavy  lines  transverse  to  the  direction  of  the  motion  of 
the  drum,  or  with  a  strip  of  newspaper  in  which  the 
printing  is  not  too  much  broken,  and  observing  through  a 
couple  of  little  windows  cut  in  a  cardboard  screen  placed 
close  before  the  drum,  one  window  being  fixated  and  the 

1  In  the  first  case,  there  is  still  another  factor  besides  the  difference  of  direct 
and  indirect  vision  ;  namely,  the  opposite  direction  of  the  movements  compared. 
The  retinal  image  of  the  non-fixated  pendulum  sweeps  over  the  retina  at  twice 
its  proper  rate,  and  underestimation  of  the  rate  of  the  fixated  pendulum,  if  any, 
would  also  accrue  to  it. 


225]  VISUAL   PERCEPTION   OF  SPACE,    ETC.        307 

other  seen  indirectly.  This  method  has  the  advantage 
of  having  the  movements  to  be  compared  take  place  in  the 
same  direction. 

Exner,  B  ;  Aubert,  C,  362  ff.  ;  Dresslar  ;  Stern,  341  ff.,  362. 

225.  Relativity  of  Movement.  In  many  cases  the  data 
for  the  perception  of  movement  are  equivocal ;  either  of 
two  interpretations  is  possible,  and  central  or  apperceptive 
conditions  determine  which  shall  prevail.  Examples  of 
this  are  frequently  found  outside  the  laboratory  in  the  case 
of  parallel  railway  trains,  or  in  fording  rapidly  flowing 
streams,  or  when  the  clouds  drive  swiftly  across  the  moon. 
Substitutes  for  these  have  been  prepared  (Mach,  A,  65  f. ; 
Budde,  131  f. ;  Wood)  ;  but  the  phenomena  are  perhaps 
sufficiently  well-known  without  further  demonstration  in 
this  connection.  In  the  following  experiment,  movement 
may  appear  to  be  divided  between  the  moving  object  and 
that  at  rest. 

a.  Move  a  pinhead  along  the  imaginary  line  C  D  in  the 
figure  below,  keeping  the  eye  constantly  fixed  on  the  pin- 
head  as  it  moves.  The  line  A  B  will  seem  to  move  down- 
ward and  to  the  left  as  the  pinhead  goes  from  D  to  (7,  and 
upward  and  to  the  right  as  it  goes  from  C  to  D.  Steady 
fixation  of  the  pinhead  is  essential;  and  a  moderate  rate 
of  movement,  which  can  be  found  by  a  few  trials,  gives  the 
best  result.  The  right  and  left  movement  of  A  B  may 
be  increased  by  moving  the  pinhead  in  a  line  more  nearly 

horizontal  than  C  D. 

C 


A- 


D 


An  oscillating  movement  of  the  line  is  to  be  observed 
when  a  compass  point  is  made  to  draw  an  imaginary  arc 


308       LABORATORY  COURSE  IN  PSYCHOLOGY.      [226 

across  it,  cutting  the  line  e  /,  for  example,  in  the  dotted 
arc  </  h  i.  As  the  point  advances  from  g  to  h  the  line 
appears  to  take  the  position  of  e '  f  ;  as  the  point  traverses 
the  region  about  h  there  is  a  change,  and  the  line  inclines 
in  the  direction  of  e"  f".  As  before,  constant  fixation  of 
the  moving  point  is  essential. 

For  a  somewhat  similar  and  still  more  striking  experi- 
ment, see  Helmholtz,  at  the  place  cited  below.  Cf.  also 
the  apparent  movements  observed  in  the  case  of  a  pinhead 
moved  over  the  Zollner  figure,  Ex.  191  I. 

Helmholtz,  A,  G.  711  ff.,  763,  Fr.  727  ff.,  786  (568  ff.,  619); 
Hering,  A,  557  ff.;  Wundt,  A,  4te  Ann.,  II.,  156  f.;  Stern,  377  ff. 


226.  Illusory  Movements  of  Objects  at  Rest.  Two  more 
or  less  distinguishable  kinds  of  illusory  movements  of  rest- 
ing objects  are  to  be  observed  ;  one  in  which  the  eyes  them- 
selves are  moved  in  some  unusual  manner,  and  the  other 
when  their  motion,  if  any,  is  slight,  and  fixation  at  least 
approximately  constant.  Examples  of  the  first  sort  haye 
been  met  in  Exs.  50  and  177 ;  and  the  general  principle 
has  been  laid  down,  that  when  the  line  of  regard  is  shifted 
voluntarily,  objects  normally  appear  at  rest,  but  when  the 
shifting  is  involuntary,  objects  seem  to  move ;  many  fur- 
ther illustrations  may  be  found  in  Hoppe,  C,  Chapter  I. 
It  is  the  second  sort,  however,  that  will  be  considered  here. 

a.  Wavering  of  Points  and  Small  Objects  under  Long 
Fixation.  Pick  out  a  small  and  isolated  speck  upon  the 
wall  or  floor,  and  fixate  it  steadily  for  a  considerable  time, 


226]  VISUAL   PERCEPTION   OF  SPACE,    ETC.        309 

but  without  straining  of  the  eyes.  After  a  while  it  will 
appear  to  move  a  little  hither  and  thither,  or  to  crawl  like 
an  insect.1  Hoppe  considers  it  due  to  slight  unconscious 
movements  of  the  eyes,  a  sort  of  tremor  of  the  eye  muscles. 
Cf.  Ex.  134.  Exner,  however,  has  been  led  by  his  experi- 
ments to  believe  it  dependent  upon  an  uncertain  and  varying 
localization  of  the  retinal  impression,  which  can,  in  case  of 
very  small  or  faint  stimuli,  reach  a  considerable  extent.2  A 
somewhat  similar  movement  appears  when  the  stars  are  fix- 
ated continuously,  and  has  long  been  known  to  astronomers. 

b.  Autokinetic  Sensations.  When  the  faint  fixated  point 
is  the  only  thing  visible  in  the  entire  field,  more  continuous 
and  extended  movements  are  to  be  observed  —  seeming  some- 
times as  great  as  20-30°.  Sensations  of  movement  of  this 
kind  have  been  called  by  Aubert  "  autokinetic  sensations." 

Arrange  the  dark  box  as  for  Ex.  178,  but  make  the  light 
point  very  faint  by  covering  the  pin-hole  with  several 
thicknesses  of  paper  on  the  outside.  Set  the  box  so  that 
the  eyes  when  fixating  the  point  shall  be  in  an  uncon- 
strained position.  Cover  the  head  and  the  top  of  the  box 
with  an  opaque  cloth  so  as  to  exclude  all  extraneous  light. 
Fixate  the  point,  and  observe  after  a  time  the  movement  in 
question.  Notice  its  extent,  and  that  it  takes  place  while 
the  fixation  is  to  all  appearances  unbroken.  Charpeiitier 
reports  several  observations  that  justify  this  subjective  im- 
pression of  fixity  of  fixation. 


1  Portions  of  the  wall  or  floor  about  the  speck  may  often  seem  to  move  with 
it,  the  movement  being  a  sort  of  slow  flowing,  something  like  that  in  the  nega- 
tive after-image  of  motion,  but  not  like  that  occurring  in  one  direction  only. 
Right  and  left  movements  have  been  chiefly  noticed  by  the  writer,  but  others 
very  probably  might  be  found. 

2  A  very  faint  retinal  stimulation  may  be  compared  in  the  wideness  of  its 
irradiation  and  the  uncertainty  of  its  location  to  a  very  faint  dermal  stirmila- 
tion  ;  it  is,  as  it  were,  a  retinal  tickle.    The  area  affected  Exner  calls  the  Circle 
of  Action  (Aktionslveis),  and  any  point  in  this  may  at.  one  time  or  another  fur- 
nish the  local  coloring  for  the  point  Been. 


310       LABORATORY  COURSE  IN  PSYCHOLOGY.     [227 

Exner's  explanation  is,  that  in  spite  of  the  fact  that  fix- 
ation is  continuously  (and  even  reflexly)  maintained,  the 
false  localization  mentioned  in  a  is  operative.  If  this  takes 
place  successively  in  one  direction,  it  brings  about  a  con- 
tinued voluntary  effort  at  fixation,  which,  while  it  causes  no 
actual  movement  of  the  eyes,  yet  gives  the  impression  of 
having  done  so. 

c.  Movements  of  Besting  Objects  viewed  with  Eyes  in 
Constrained  Positions.  Fixate  the  point  of  light  as  in  b, 
but  arrange  the  box  so  that  in  doing  so  the  eyes  shall 
be  turned  strongly  upward  or  to  one  side.  Maintain  the 
fixation  steadily,  and  after  a  few  seconds  the  point  will 
appear  to  be  in  motion  in  the  direction  in  which  the  eyes 
are  turned.  The  apparent  motion  is  due  to  the  growing 
fatigue  (and  perhaps  to  a  partial,  though  unintentional, 
relaxation  of  the  muscles),  which  is  continuously  met  by 
voluntary  efforts  of  fixation.  The  experiment  can  be  made 
even  more  satisfactorily  on  a  very  small  gas  flame  in 
a  dark  room.  It  shows  both  the  tendency  to  take  intended 
movements  for  actual  ones,  and  the  dulness  of  the  kinses- 
thetic  sensations  of  the  eyes  in  that  they  do  not  reveal  the 
true  condition  of  things.1 

Charpentier  ;  Aubert,  C ;  Exner,  A,  and  the  literature  cited  by 
him.  On  a,  Hoppe,  C,  §  1  ;  Exner,  A.  On  c,  Hering  (Hillebrand, 
B,  150). 

227.  Illusions  of  Form  depending  on  False  Estimation 
of  the  Bate  of  Motion.  Zollner's  Anorthoscopic  Illusions. 

a.  In  the  middle  of  a  sheet  of  stiff  paper  cut  a  slit  a 
couple  of  inches  long  by  an  eighth  of  an  inch  wide.  On 

1  This  apparent  movement  is  of  the  same  general  nature  as  that  observed  by 
patients  suffering  from  paralysis  of  the  external  rectus  muscle  of  the  eye.  An 
experiment  designed  to  imitate  their  condition  more  exactly  is  given  by  Mach, 
A,  57  (James,  II.,  509) ;  but,  like  Professor  James,  the  writer  has  not  been  suc- 
cessful in  attempting  to  repeat  the  experiment. 


228]         VISUAL  PERCEPTION  OF  SPACE,  ETC.         311 

another  piece  draw  a  heavy  black  circle  an  inch  in  diameter. 
Place  the  second  sheet  against  the  back  of  the  first,  and 
move  it  rapidly  from  side  to  side  in  a  direction  transverse 
to  that  of  the  slit,  in  such  a  way  that  the  circle  may  pass 
completely  across  behind  the  slit,  and  be  seen  through  it. 
It  will  appear  as  a  narrow  ellipse,  with  its  short  axis  lying 
in  the  direction  of  movement.1 

b.  Eepeat  the  experiment  just  made,  but  this  time  move 
the  circle  very  slowly.  The  result  will  be  an  apparent  dis- 
tortion in  the  contrary  direction.  These  illusions  hold 
equally  well  with  figures  other  than  the  circle.  In  both 
cases  the  distortion  appears  to  depend  on  a  false  estimate 
of  the  rate  of  motion,  similar  to  that  found  for  touch  in 
Ex.  12. 

Zollner,  C ;  Helmholtz,  A,  G.  498  ff.,  749,  Fr.  465  ff.,  770  (352  ff., 
605  f.)  ;  Hering,  A,  559  ff. 

228.  Perceptive  Inference  of  Motion.  By  this  term  is 
meant  such  perception  of  movement  as  takes  place  when 
no  continuous  movement  is  presented,  but  merely  isolated 
phases  of  one.  Its  type  is  the  apparent  movement  seen 
in  the  stroboscope,  zoetrope,  and  similar  instruments.2  It 
stands  midway  between  the  cases  in  which  movement  is 
directly  perceived  and  those  in  which  it  is  entirely  infer- 
ential ;  the  perception  depends  on  separate  phases,  but 
remains  nearly  or  quite  at  the  level  of  perception. 

a.  With  the  instrument  at  hand  observe  the  conditions 
required  to  produce  the  appearance  of  simple  movement  of 

1  This  illusion  of  Zollner's  is  to  be  carefully  distinguished  from  the  ordinary 
anorthoscopic  illusion  of  Plateau,  in  which  the  slit  moves  at  the  same  time  as 
the  diagram,  and  in  a  contrary  direction.    In  the  latter  the  distortion  can  be 
accounted  for  simply  as  a  matter  of  positive  after-images.    For  information  on 
the  Plateau  illusion,  consult  the  references  at  the  end  of  the  experiment. 

2  The  more  perfect  instruments  of  this  kind,  the  kinetoscope,  vitascope,  etc., 
are  of  less  psychological  interest,  because  the  phases  presented  are  probably 
indistinguishable  as  phases,  even  under  the  most  favorable  circumstances. 


312       LABORATORY  COURSE  IN  PSYCHOLOGY.     [228 

the  figures  in  their  own  stations,  and  of  translation  in  the 
direction  of  the  rotation  of  the  instrument  and  in  the  re- 
verse direction.  Set  the  instrument  in  rapid  movement, 
and  observe  the  superposed  positive  after-images  of  several 
phases,  noticing  that  the  presence  of  several  at  the  same 
time  interferes  with  the  perception  of  the  movement. 

For  quantitative  studies  of  some  of  the  conditions  of 
this  illusion  see  Otto  Fischer,  and  for  notes  on  the  history 
and  applications  of  the  stroboscope  see  both  Fischer  and 
Grutzner. 

b.  When  important  phases  are  wanting,  the  perception 
of  the  whole  is  often  but  little  disturbed.  This  may  be 
tried  with  almost  any  set  of  zoe trope  pictures,  but  will 
probably  succeed  best  with  one  representing  a  familiar 
movement ;  for  example,  in  a  strip  representing  a  gym- 
nast turning  a  somersault  through  a  paper  circle  held  by  a 
second  figure.  The  figure  of  the  gymnast  may  be  removed 
from  three,  or  even  four,  successive  pictures  when  the 
whole  number  is  only  thirteen,  without  abolishing  the  per- 
ception of  the  movement.  Those  covered  should  be  such 
as  show  the  figure  passing  through  the  hoop ;  those  re- 
tained should  show  the  beginning  and  end  of  the  leap. 
Careful  observation,  however,  will  make  clear  that  the  gym- 
nast disappears  for  an  instant,  and  there  is  also  a  certain 
liability  that  the  original  conception  may  be  supplanted  by 
another,  i.e.,  that  the  phases  may  be  given  a  different  inter- 
pretation ;  in  this  case,  that  the  gymnast  may  seem  to  dis- 
appear into  the  hoop,  and  then  reappear  again  on  the  same 
side,  as  he  might  if  the  covering  of  the  hoop  were  elastic 
and  he  were  thrown  back  by  it. 

A  convenient  way  to  remove  the  figure  is  to  cut  a  strip 
of  paper  a  couple  of  inches  wider  than  the  zoetrope  strip, 
and  long  enough  to  cover  three  or  four  pictures.  Lay  this 
paper  strip  on  the  picture  strip,  and  fold  the  overlapping 


229]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         313 

edges  around  the  latter,  and  crease  them  back  so  that  the 
paper  will  hold  its  place  on  the  picture  strip.  Then  trace 
the  parts  of  the  pictures  to  be  retained,  line  them  in,  and 
color  them  to  match  the  rest  of  the  band.  Griitzner  used 
with  success  the  figures  of  boys  playing  leap-frog. 

Helmholtz,  A,  G.  494  ff.,  Fr.  461  ff.  (349  ff.);  O.  Fischer;  Wundt, 
A,  4te  Aufl.,  II.,  159  f.;  Griitzner. 

229.  Thompson's  Strobic  Circles.  The  illusory  motion 
in  these  well-known  figures  depends  upon  positive  after- 
images, but  in  a  way  quite  different  from  that  noticed  in 
Ex.  223.  The  important  point  here  is  the  blurring  to 
which  they  give  rise. 


a.  Concentric  Circles.  Give  to  the  diagram  above  a 
circular  motion  in  its  own  plane,  like  that  given  to  a  vessel 
when  rinsing  it.  The  radius  of  the  circle  in  which  the 
movement  is  made  should  be  quite  small,  and  the  rate  2-4 
circuits  per  second.  Observe  the  apparent  rotation  of  the 
circles  (or,  more  exactly,  of.  &  couple  of  relatively  clear  cut 


OF  THE 


314       LABORATORY   COURSE  IN  PSYCHOLOGY.     [229 

sectors),  which  takes  place  at  the  same  rate  and  in  the 
same  direction  as  the  movement  given  the  diagram.1 

As  has  been  said,  the  illusion  is  a  matter  of  positive 
after-images.  Suppose  the  diagram  to  be  given  a  right 
and  left  movement  only,  in  extent  equal  to  the  breadth  of 
one  ring.  It  is  clear  that  any  persistence  of  the  images 
will  tend  to  blur  the  parts  of  the  circles  most  nearly  per- 
pendicular to  the  line  of  movement  (the  vertical  parts), 
while  it  will  not  affect  at  all  those  parts  most  nearly  par- 
allel to  that  line  (the  horizontal  parts),  which  will  conse- 
quently remain  clear  cut.  Thus  arise  the  above-mentioned 
sectors.  If  the  movement  is  not  quite  right  and.  left,  but 
a  little  inclined  toward  the  body  at  the  right,  it  is  evident 
that  the  regions  most  clear  and  most  blurred  will  lie  in  a 
little  different  position  from  that  before  occupied,  and  in 
general  that  as  the  direction  of  the  movement  is  changed 
the  position  of  the  sectors  will  be  correspondingly  changed, 
and,  further,  that  as  movement  in  a  circle  is  movement 
with  a  continual  change  in  direction,  the  positions  of  the 
sectors  will  also  change  continuously.  In  one  complete 
movement  of  the  diagram  through  its  circuit  the  sectors 
will  also  have  occupied  once  every  position  in  the  concen- 
tric circles,  and  the  rates  will  therefore  be  the  same. 

Movements  of  the  eyes  may  produce  the  same  rotation 
with  a  diagram  at  rest.  This  is  easiest  to  get  ivhen  some- 
thing is  moved  rather  slowly  close  to  or  actually  on  the 
surface  of  the  diagram,  and  carefully  followed  with  the 
eye.  The  illusion  is  very  strong  in  indirect  vision,  and 
movements  of  the  eyes  in  observing  a  moving  diagram  in 
the  hand  will  often  set  going  others  seen  indirectly  and 
actually  at  rest. 

1  The  sectors  may  resemble  somewhat  those  seen  by  the  astigmatic  eye,  but 
are  not  due  to  that  defect.  Astigmatism,  however,  complicates  the  appearance, 
and  the  description  which  follows  would  apply  strictly  only  to  the  perfect  eye. 


229]         VISUAL  PERCEPTION   OF  SPACE,  ETC.         315 

b.  Cog  Wheels.  Give  to  the  diagram  below  a  move- 
ment similar  to  that  used  in  a,  but  of  not  too  great  extent, 
and  observe  in  the  wheel  at  the  left  a  slow  retrograde 
movement.  In  that  at  the  right,  Bowditch  and  Hall  re- 


port a  still  slower  rotation  in  the  same  direction  as  that  of 
the  diagram  ;  the  writer,  however,  has  had  poor  success 
in  getting  this  wheel  to  move.1  The  same  effect  may  be 
observed  with  rack-work  like  that  below  when  given  the 
same  rinsing  motion,  the  upper  rack  moving  to  the  left 
and  the  lower  to  the  right  when  the  rinsing  movement  is 
like  that  of  the  hands  of  a  watch. 

The  explanation  that  has  been  offered  for  these  figures, 


while  perhaps  deficient  in  some  details,  covers  the  chief 
phenomenon  well  enough.  It  is  simplest  in  case  of  the 
rack-work,  and  rests  upon  two  probable  assumptions, 

1  These  authors  find  a  tendency  to  retrograde  rotation  in  the  wheel  with  out- 
ward cogs  when  the  rate  and  extent  of  the  movement  of  the  diagram  are  con- 
siderable. This  seems  to  be  easier  to  get,  and  something  of  the  same  kind  can 
be  observed  with  the  rayed  figure  of  Ex.  109  a. 


316       LABORATORY  COURSE  IN  PSYCHOLOGY.     [229 

namely,  that  the  movement  is  judged  from  the  cogs  rather 
than  from  the  bar  of  the  rack-work,  and  that  the  direction 
of  the  illusory  movement  will  be  suggested  by  that  of  the 
actual  movement  at  such  times  as  the  cogs  are  most  clearly 
seen.  Suppose  the  diagram  to  be  in  motion  in  the  way  de- 
scribed, the  direction  being  that  of  the  hands  of  a  watch ; 
and  suppose,  further,  that  the  upper  rack  at  the  instant 
under  consideration  is  at  the  uppermost  point  of  its  circuit. 
Its  movement  for  the  instant  just  past  has  been  upward 
and  to  the  right,  and  the  cogs  have  been  rising  into  portions 
of  the  field  occupied  just  before  by  the  bar,  and  are  there- 
fore confused  with  its  after-image,  and  not  plainly  seen. 
The  cogs  on  the  lower  rack,  however,  have  been  advancing 
into  a  new  part  of  the  field,  have  been  seen  clearly,  and  in 
movement  towards  the  right ;  i.e.,  in  the  direction  of  the 
rinsing.  These  conditions  continue  more  or  less  unchanged 
during  the  instant  following  the -point  of  greatest  upward 
excursion.  As  the  movement  of  the  diagram  progresses, 
however,  the  cogs  of  the  upper  rack  begin  to  advance  into 
a  fresh  part  of  the  field,  and  those  of  the  lower  rack  to 
retreat  into  the  after-image  of  their  bar  ;  and  this  continues 
till  the  lowest  point  of  the  circuit  is  nearly  reached,  when 
the  cogs  of  the  upper  rack  are  seen  clearly,  and  in  motion 
toward  the  left,  i.e.,  in  an  opposite  direction  to  the  rinsing. 
In  brief,  when  the  upper  rack-work  is  seen  clearest  it  is 
moving  to  the  left,  and  when  the  lower  is  seen  clearest  it 
is  moving  toward  the  right ;  and  from  these  brief  but  clear 
observations  the  continuous  movement  is  inferred.  The 
racks  as  here  represented  correspond  to  the  upper  and 
lower  parts  of  the  wheel  with  inside  cogs ;  and  what  was 
true  of  the  racks  at  the  upper  and  lower  points  of  their 
circuit  is  true  of  some  part  of  the  circumference  of  the 
wheel  all  the  time,  —  of  that  part,  namely,  whose  cogs  are 
at  the  instant  most  distinctly  visible.  If  the  upper  and 


229]         VISUAL   PERCEPTION  OF  SPACE,    ETC.         317 

lower  racks  were  interchanged,  so  that  the  teeth  pointed 
outward,  they  would  correspond  to  the  wheel  with  outside 


°°0 


cogs  ;  and  the  upper  one  ought,  if  the  same  principle  holds, 
to  give  movement  in  the  direction  of  that  of  the  diagram, 
the  lower  in  the  contrary  direction. 


318      LABORATORY  COURSE  IN  PSYCHOLOGY.     [230 

c.  The  first  of  the  diagrams  above  (p.  317)  shows  a  com- 
bination of  the  movements  of  both  a  and  b.  The  second 
and  third  show  the  increasing  difficulty  with  decreasing 
number  of  teeth  in  the  wheels.  For  yet  other  special 
observations,  see  the  paper  of  Bowditch  and  Hall. 

Thompson,  B  ;  Bowditch  and  Hall.  Also  the  Scientific  Ameri- 
can, XLL,  1879,  85,  133. 

230.  Chromatokinopsia.  Experiment  of  the  "Flutter- 
ing Heart."  This  experiment,  like  the  last,  depends  on 
positive  after-images,  but  of  a  peculiar  kind.  The  experi- 
ment takes  its  name  from  the  figures  with  which  it  was  at 
first  performed,  but  others  answer  equally  well. 

a.   Prepare  a  diagram  like  that  shown  in  the  cut  below, 


making  the   ground  of  red  paper,  the  rings  of  blue   of 
about  equal  brightness,  the  little  circles  of  a  variety  of 


230]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         319 

other  colors,  and  the  heavy  lines  in  black  ink.  Experi- 
ment in  a  dark  room  (after  allowing  five  or  ten  minutes 
for  adaptation  of  the  eye)  or  at  night. 

Use  a  small  gas  or  candle  flame  for  illumination,  holding 
the  diagram  a  yard  or  two  from  the  flame,  and  varying  the 
distance  if  the  effect  is  not  secured.  Give  the  diagram  a 
short  side  to  side  motion  in  its  own  plane,  moving  it  three 
or  four  times  a  second,  and  notice  an  apparent  slipping  or 
springing  of  the  rings  from  side  to  side.  Close  observa- 
tion of  the  small  circles  of  green  or  blue  will  show  that  it 
is  not  so  much  the  colored  figures  themselves  that  move,  as 
it  is  a  whitish  shimmering  image  that  seems  to  rest  upon 
them,  and  to  hang  back  when  the  diagram  is  moved. 
Notice  that  the  apparent  movement  is  more  marked  in 
indirect  vision.  A  convenient  diagram  for  showing  this  is 
made  by  pasting  a  row  of  little  blue  circles  on  a  strip  of 
red  cardboard.  Some  slipping  of  the  remotest  circles  may 
be  noticed,  even  when  the  diagram  is  shaken  in  a  place  no 
darker  than  a  well-shadowed  corner  of  the  room.  Observe, 
on  the  small  circles,  that  all  colors  do  not  show  the  phenom- 
enon equally  well.  The  general  effect  is  the  same  when 
the  colors  of  the  ground  and  the  rings  are  reversed  (blue 
ground,  red  rings)  ;  but  the  whitish  image  is  no  longer  to 
be  seen,  and  the  parts  of  the  rings  at  right  angles  to  the 
direction  of  motion  are  darkened.  With  this  combination 
of  colors  white  cross-lines  should  be  used  instead  of  black. 
The  black  or  white  lines  are  relatively  fixed,  and  thus  ren- 
der the  apparent  movement  of  the  colors  more  easily  dis- 
cernible.1 

b.    Gray  Figures  on  a  Colored  Ground  and  Colored  Fig- 


1  An  early  explanation  of  the  illusion  referred  it  to  chromatic  aberration, 
and  has  recently  been  revived  in  more  developed  form.  While  it  is  probably 
a  co-operating  factor,  it  is  certainly  not  the  sole,  nor  even  the  most  important, 
one. 


320       LABORATORY  COURSE  IN  PSYCHOLOGY.      [230 

ures  on  a  Gray  Ground  show  the  same  slipping.  The  gray 
and  the  color  should  be  about  equally  bright.  If  gray  rings 
are  not  at  hand,  prepare  a  diagram  like  that  below  by  past- 
ing on  the  colored  ground  gray  strips  about  2.5  mm.  wide, 
and  ruling  heavy  black  lines  across.1  Observe  in  a  dark 


room  as  before.     Try  also  with  diagrams  in  which  the 
ground  is  gray  and  the  figures  colored. 

c.  Gray  Figures  on  a  Black  Ground.  On  a  ground  of 
black  velvet  fasten  a  small  circle  of  black  cardboard  (e.g., 
about  4  cm.  in  diameter),  and  concentric  with  it  a  small 
circle  of  white  paper  (about  1  cm.  in  diameter).  A  jelly- 
like  slipping  of  the  cardboard  will  be  observed  when  the 
diagram  is  shaken.  Stronger  light  is  required  for  this 
experiment  than  for  most  of  the  previous  ones,  a  tolerable 
result  being  obtainable  in  a  well-shadowed  corner  of  the 
room  by  daylight.  The  character  of  the  illusory  move- 
ment also  seems  different.  It  is  hardly  necessary  .to  say 
that  "  black  "  cardboard  is  really  a  very  dark  gray.  It  is 


1  If  the  ground  is  blue,  use  white  lines  instead  of  hlack  ;  for  on  the  blue  groum 
black  lines  themselves  may  seem  to  move. 


232]         VISUAL  PERCEPTION  OF  SPACE,    ETC.         321 

possible  under  favorable  circumstances  to  get  a  slipping 
even  of  a  grille  work  of  white  paper  on  a  background  of 
black  velvet. 

Experiments  b  and  c  exclude  chromatic  aberration,  and 
favor  an  explanation  depending  in  some  way  on  retinal 
inertia.  The  apparent  slipping  is  due  to  a  lag  in  the  reti- 
nal response  —  to  a  delayed  after-image,  as  it  were,  but 
something  different  evidently  from  the  ordinary  after-image. 
The  flickering  in  a  and  b  is  regarded  by  Szili  as  a  contrast 
phenomenon.  The  matter  deserves  further  investigation. 

Wheatstone,  C ;  Helmholtz,  A,  G.  533  f .,  Fr.  504  (383)  ;  Mayer- 
hausen  ;  Szili,  A  and  B  ;  Schapringer. 

231.  Equivocal  Movement  Depending  on  Equivocal  Re- 
lief.    Sinsteden  observed  this  upon  windmills ;  but  the  ex- 
periment can  easily  be  made  in  the  laboratory  by  setting 
the  disk  used  in  Ex.  223  b  in  slow  rotation,  and  viewing 
it  with  a  single  eye  from  a  position  several  yards  distant 
and  nearly  in  its  own  plane.     If  the  disk  actually  faces 
the  left,  and  the  movement  is  like  that  of  the  hands  of  a 
watch,  a  slight  effort  only  will  be  necessary  to  make  it 
seem  to  face  the  right  with  a  contrary  movement.     For  still 
other  illusions  affecting  rotations,  see  the  brief  paper  of 
Nichols,  and  Mach,  A,  99  f.,  102. 

Helmholtz,  A,  G.  770,  Fr.  795  (626). 

• 
VISUAL  SIMILARITY  AND   SYMMETRY. 

These  topics  lead  at  once  to  questions  of  aesthetics  which 
lie  beyond  the  scope  of  the  present  chapter.  They  throw 
light,  however,  upon  the  general  question  of  the  visual  per- 
ception of  figure,  and  are  treated  here  for  that  reason.  In- 
formation on  their  psycho-physiology  is  owed  chiefly  to 
Mach. 

232.  Visual   Similarity.     Figures   are   alike  for   vision, 


822       LABORATORY  COURSE  IN   PSYCHOLOGY.    [233 

i.e.,  look  alike,  when  they  present  equal  extents  in  like  di- 
rections. If  the  first  condition  is  not  fulfilled,  the  figures 
are  seen  to  be  similar,  but  different  in  size ;  if  the  second  is 
not  fulfilled,  they  may  be  known  to  be  alike,  but  are  not 
immediately  seen  to  be  so.  This  will  be  evident  from  the 
figures  below.  It  is  only  after  thinking  of  how  the  cen- 
tral square  would  look  from  a  position  45°  to  the  right  or 
left,  or  perhaps  after  still  more  complex  mental  operations, 
that  we  convince  ourselves  of  the  identity  of  the  figures. 
In  much  the  same  way  the  similarity  of  perspective  figures 


is  recognized,  e.g.,  that  of  the  sides  of  the  cube  in  Ex.  188 
b ;  in  both  instances  the  perceptive  process  is  evidently 
not  at  its  simplest. 

These  requirements  of  visual  similarity  in  the  last 
analysis  lie  chiefly  in  the  likeness  of  retinal  images  and 
eye-movements,  perhaps  also  in  part  in  the  likeness  of  the 
kinsesthetic  sensations  of  the  hands  in  touching  or  tracing 
similar  figures. 

Mach,  A,  43  ff. 

233.    Visual   Symmetry.     Symmetry   is   similarity   of  a 


233]        VISUAL  PERCEPTION  OF  SPACE,   ETC.         823 

special  kind  —  likeness  of  extent  from  a  particular  line  or 
point.  It  is  most  common  and  striking  when  the  equal 
extents  lie  on  either  side  the  median  plane,  as  in  the  fig- 
ures below.1 


The  same  is  to  be  observed  almost  without  limit  in  arch- 
itectural arid  other  decorations,  and  even  holds,  as  Soret 
shows,  with  simple  lines.  The  first  and  third  of  these  fig- 
ures give  a  distinctly  different  impression  from  the  second 
and  fourth. 


The  extreme  likeness  of  right  and  left  directions  is  re- 
sponsible for  the  mistakes  of  children  with  p  and  q  and  b 
and  d,  and  for  the  elaborateness  of  the  process  through 
which  some  adults  must  go  to  tell  which  hand  is  right  and 
which  left. 


1  All  the  figures  of  this  experiment,  except  the  straight-line  figures  next  fol- 
lowing, are  Japanese  coats-of-arms  taken  from  the  Annales  du  Musie  Guimet, 
B'Mlothcque  d' Etudes,  T.  III.,—  Coffre  d  Tresor  attribv£  au  Shogoun  lyt-Yoshi 
(1838-1853),  fyude  hfraldique  et  historique,  par  de  Millout  et  Kawamoura, 
Paris,  1896. 


324       LABORATORY  COURSE  IN  PSYCHOLOGY.     [233 

Pure  cases  of  symmetry  about  other  axes  than  the  verti- 
cal are  sometimes  found,  but  they  are  less  common  and  less 
simple  in  manner  of  perception.  Combined  vertical  and 
horizontal  symmetry  is,  however,  by  no  means  uncommon. 


Symmetry  with  reference  to  a  single  point  —  centric 
symmetry  —  is  not  uncommon,  especially  when  combined 
with  symmetry  about  the  vertical  axis. 

The  letters  of  the  alphabet,  as  Mach  has  noticed,  show 
symmetry  of  several  sorts  in  their  general  plan  :  About  a 
vertical  axis,  AHIMTUVWXY;  about  a  horizontal  axis, 
B  C  D  E  H  I  K  X ;  about  a  centre,  N  S  Z.  O  shows  symmetry 
of  all  sorts,  and  F  G  J  L  P  Q  R  are  asymmetric. 


The  importance  of  eye-movements,  and  of  the  symmetry 
of  the  visual  apparatus  itself,  is  clearer  perhaps  in  the 
case  of  symmetry  than  of  similarity ;  but  it  must  not  be 
overlooked  that  not  only  our  own  persons,  but  nearly  all 
the  world  besides,  is  symmetrical  in  plan. 

Mach,  A,  45  ff.  ;  Soret ;  Pierce. 


VISUAL  PERCEPTION  OF  SPACE,   ETC.         325 


BIBLIOGRAPHY. 

ABBER  :  Ueber  die  Bedeutung  der  Convergenz-  und  Accommodations- 
bewegungen  fiir  die  Tiefenwahrnehmung,  Wundt's  Philos.  Studien, 
XIII.,  1896-97,  116-161,  222-304. 

AUBERT:    A.   Grundziige  der  physiologischen  Optik,  Leipzig,  1876. 

B.  Physiologie  der  Netzhaut,  Breslau,  1865. 

C.  Die  Bewegungsempfindung,  Pfliiger's  Archiv,  XXXIX.,  1886, 
347-370;  XL.,  1887,  459^180,  623. 

AUERBACH  :  Erklarung  der  Brentanoschen  optischen  Tauschung,  Zeit- 
schriftfilr  Psychologic,  VII.,  1894,  152-160. 

BALDWIN:  The  Effect  of  Size-contrast  upon  Judgments  of  Position  in 
the  Retinal  Field,  Psychological  Review,  II.,  1895,  244-259.  See  also 
Science,  N.S.  IV.,  1896,  794-796. 

BEAUNIS  :   Nouveaux  Elements  de  Physiologie  Humaine,  Paris,  1888. 

VON  BEZOLD:   A.  The  Theory  of  Color,  Boston,  1876. 

B.  Eine  perspectivische  Tauschung,  Wiedemann's  Annalen,  XXIII., 
1884,  351-352. 

VAN  BIEBVLIET:  Nouvelles  mesures  des  illusions  visuelles  chez  les 
adultes  et  les  enfants,  Revue  philosophique,  XLI.,  Jan.-Juin,  1896, 
169-181. 

BINET  :  La  mesure  des  illusions  visuelles  chez  les  enfants,  Revue  phi- 
losophique, XL.,  Juillet-De'c.,  1895,  11-25. 

BOUBDON:  Experiences  sur  la  perception  visuelle  de  la  profondeur, 
Revue  philosophique,  XLIIL,  Jan.-Juin,  1897,  29-^55. 

BOWDITCH  :  "  Vision  "  in  An  American  Text-Book  of  Physiology,  Phila- 
delphia, 1896. 

BOWDITCH  AND  HALL  :  Optical  Illusions  of  Motion,  Journal  of  Physi- 
ology, III.,  1880-82,  297-307. 

BOWDITCH  AND  SOUTHABD  :  A  Comparison  of  Sight  and  Touch,  Journal 
of  Physiology,  III.,  1880-82,  232-245. 

BBENTANO:  A.  Ueber  ein  optisches  Paradoxon,  Zeitschrift  fur  Psy- 
chologie,  III.,  1892,  349-358;  V.,  1893,  61-82. 

B.   Zur  Lehre  von  den  optischen  Tauschungen,  ibid.,  VI.,  1893-94, 
1-7. 

BBEWSTEB  :  A.  The  Stereoscope,  its  History,  Theory,  and  Construction, 
with  its  application  to  the  fine  and  useful  arts  and  to  education, 
London,  1856,  pp.  iv.  235.  Contains  a  chapter  on  the  early  history 
of  the  theory  of  binocular  vision  and  the  stereoscope. 
B.  On  the  Conversion  of  Relief  by  Inverted  Vision,  Phil.  Mag.,  Ser. 
3,  XXX.,  Jan.-June,  1847,  432-137.  Contained  also  in  the  work  on 
the  stereoscope. 


826       LABORATORY  COURSE  IN  PSYCHOLOGY. 

BRUCKE:   Vorlesungen  iiber  Physiologie,  4te  Aufl.,  Wien,  1885-1887. 
BRTJNOT:   Les  illusions  d'optique,  Revue  scientiftque,  LIT.,  1893,  210-212. 
BUDDE  :   Ueber  metakinetische  Scheinbewegungeii  und  iiber  die  Wahr- 

nehmung  der  Bewegung,  Du  Bois-Reymond's  Archiv,  1884,  127-152. 
BURMESTER:    Beitrag  zur  experimentellen  Bestimmung  geometrisch- 

optischer  Tauscbungen,  Zeitschrift  fur  Psychologic,  XII.,  1896,  355- 

394.     On  Poggendorff's  illusion. 
CHARPENTIER:   Sur  uiie  illusion  visuelle,  Comptes  rendus,  CII.,  1886, 

1155-1157.     See  also,  Nouveaux  faits  a  propos  du  "  balancement  des 

e'toiles,"  ibid.,  1462-1464. 
DELBOEUF:    A.   Note  sur  certaines  illusions  d'optique,  Bull,  de  I'Acad. 

roy.  de  Belgique,  2e  se'rie,  XIX.,  1865,  No.  2,  195-216. 

B.  Seconde  note  sur  de  nouvelles  illusions  d'optique,  ibid.,  XX., 
1865,  No.  6,  70-97. 

C.  Sur  une  nouvelle  illusion  d'optique,  Revue  scientiftque,  LI.,  1893, 
237-241. 

DIXON  :  On  the  Relation  of  Accommodation  and  Convergence  to  our 
Sense  of  Depth,  Mind,  N.S.  IV.,  1895,  195-212. 

DRESSLAR  :  A  New  Illusion  for  Touch  and  an  Explanation  for  the  Illu- 
sion of  Displacement  of  Certain  Cross  Lines  in  Vision,  American 
Journal  of  Psychology,  VI.,  1893-95,  275. 

A  New  and  Simple  Method  for  Comparing  the  Perception  of  Rate  of 
Movement  in  the  Direct  and  Indirect  Fields  of  Vision,  ibid.,  312. 

Du  BOIS-REYMOND,  C.:  Ueber  Briickes  Theorie  des  korperlichen 
Sehens,  Zeitschrift  fur  Psychologic,  II.,  1891,  427-437. 

DVORAK:   Ueber  Analoga  der  personlichen  Differenz  zwischen  beiden 
Augen  und  den  Netzhautstellen  desselben  Auges,  Sitz.-ber.  d.  k. 
bohm.  Gesells.  d.  Wis.  in  Prag ;  Jahrgang,  1872,  Jan.-Juni,  pp.  65-74. 
See  also  American  Journal  of  Psychology,  VI.,  1893-95,  575  ff. 

EINTHOVEN  :  On  the  Production  of  Shadow  and  Perspective  Effects  by 
Difference  of  Colour,  Brain,  XVI.,  Pts.  Ixi.,  and  Ixii.,  1893,  191-202. 

EXNER:  A.  Ueber  autokinetische  Empfmdungen,  Zeitschrift  fur  Psy- 
chologic, XII.,  1896,  313-330. 

B.  Ein  Versuch  iiber  die  Netzhautperipherie  als  Organ  zur  Wahr- 
nehmung  von  Bewegungen,  Pfluger's  Archiv,  XXXVIII.,  1886,  217- 
218.  See  also  several  of  the  papers  of  Exner  cited  after  Chap.  V. 

FECHNER:  Elemente  der  Psychophysik,  Zweite  unveranderte  Auflage, 
Leipzig,  1889. 

FILEHNE:  Die  Form  des  Himrnelsgewolbes,  Pfluger's  Archiv,  LIX., 
1894,  279-308.  Contains  a  brief  historical  account  of  the  older  lit- 
erature of  the  question. 

FISCHER,  O:  Psychologische  Analyse  der  stroboskopiscben  Erscbeinun- 
gen,  Wundt's  Philos.  Studien,  III.,  1886,  128-156. 


VISUAL   PERCEPTION  OF  SPACE,   ETC.         327 

FISCHER,  R. :  Griissenschatzungen  im  Gesichtsfeld,  von  Graefe's  Ar- 
chiv,  XXXVII.,  1891,  i.,  97-136;  iii.,  55-S5. 

VON  FLEISCHL  :  Physiologisch-optische  Notizen  (2te  Mittheilung),  Sitz.- 
ber.  d.  k.  Akademie  d.  Wiss.  i.  Wien,  math.-nat.  Classe,  LXXXVI., 
1882,  Abth.  iii.,  8-25. 

FRANKLIN,  CHRISTINE  LADD.:  A  Method  for  the  Experimental  Deter- 
mination of  the  Horopter,  American  Journal  of  Psychology,  I., 
1887-88,  99-111.  See  also  Science,  N.S.  III.,  1896,  274. 

GREEFF  :  Untersuchungen  iiber  binokulares  Sehen  mit  Anwendung  des 
Heringschen  Fallversuchs,  Zeitschrift  fur  Psychologie,  III.,  1891- 
92,  21-47. 

GRUTZNER  :  Einige  Versuche  mit  der  Wunderscheibe,  Pfliiger's  Archiv, 
LV.,  1893-94,  508-520. 

GUYE  :  L'illusion  d'optique  dans  la  figure  de  Zollner,  Revue  scientijique, 
LI.,  1893,  593-594. 

HELMHOLTZ  :   A.  Hahdbuch  der  physiologischen  Optik,  2te  Aufl.,  Ham- 
burg uud  Leipzig,  1886-96.     French  translation  of  the  first  edition : 
Optique  physiologique,  Paris,  1867. 
B.   Popular  Scientific  Lectures,  First  Series,  New  York,  1885. 

HERING:    A.   Der  Raumsinn   und   die  Bewegungen  des  Auges,   Her- 
mann's Handbuch  der  Physiologie,  III.,  Th.  i.,  343-601. 
B.  Beitrage  zur  Physiologie,  Leipzig,  1861-64. 

HEYMANS:   A.  Quantitative  Untersuchungen  iiber  das  "optische  Para- 
doxon,"  Zeitschrift  fur  Psychologie,  IX.,  1895-96,  221-255. 
B.  Quantitative  Untersuchungen  iiber  die  Zollnersche  und  die  Loeb- 
sche  Tauschung,  ibid.,  XIV.,  1897,  101-139. 

HILLEBRAND  :  A.  Die  Stabilitat  der  Raumwerte  auf  der  Netzhaut,  Zeit- 
schrift fur  Psychologie,  V.,  1893,  1-60. 

B.  Das  Verhaltnis  von  Accommodation  und  Konvergenz  zur  Tiefen- 
lokalisation,  ibid.,  VII.,  1894,  97-151. 

HOFLER:   Kriimmungskontrast,   Zeitschrift  fur  Psychologie,  X.,  1896, 

99-108. 
HOLTZ  :   Ueber  den  unmittelbaren  Grosseneindruck  in  seiner  Beziehung 

zur  Entfernung  und  zum  Contrast,  Gottinger  Nachrichten,  1893, 159- 

167,  496-504. 
HOPKINS  :    Experimental  Science,  New  York,  1890.    Also  article  in  the 

Scientific  American,  LXIII.,  1890,  406. 

HOPPE,  J.  I.:    A.  Psychologisch-physiologische  Optik,  Leipzig,  1881. 

B.  Beitrag  zur  Erklarung  des  Erheben-  und  Vertieft-Sehens,  Pflii- 
ger's  Archiv,  XL.,  1887,  523-532. 

C.  Die  Schein-Bewegungen,  Wurzburg,  1879,  pp.  xii..  212. 


328      LABORATORY  COURSE  IN  PSYCHOLOGY.. 

HYSLOP:   A.  On  Wundt's  Theory  of  Psychic  Synthesis  in  Vision,  Mind, 

Ser.  1,  XIII.,  1888,  499-526.     See  also  other  articles  by  Hyslop,  ibid., 

XIV.,  1889,  393-401,  and  XVI.,  1891,  54-79. 

B.  Experiments  in  Space  Perception,  Psychological  Review,  I.,  1894, 

257-273,  581-601. 

JAMES  :   Principles  of  Psychology,  New  York,  1890. 
JASTROW:   A.  A  Study  of  Zollner's  Figure  and  Other  Related  Illusions, 

American  Journal  of  Psychology,  IV.,  1891-92,  381-398.    See  also 

abstract  in  Nature,  XLVL,  1892,  590-592,  and  Eevue  scientiflque,  L., 

1892,  68^-692. 

B.  On  the  Judgment  of  Angles  and  Positions  of  Lines,  American 

Journal  of  Psychology,  V.,  1892-93,  214-223. 
JUDD:   Some  Facts  of  Binocular  Vision,  Psychological  Review,  IV., 

1897,  374-389. 
KIBSCHMANN:    Die  Parallaxe  des  indirecten  Sehens  und  die  spaltfor- 

migen  Pupillen der  Katze,  Wundt's  Philos.  Studien,  IX.,  1893-94, 447- 

495. 

KNOX  AND  WATANABE  :  On  the  Quantitative  Determination  of  an  Op- 
tical Illusion,  American  Journal  of  Psychology,  VI.,  1893-95,  413-421, 

509-514. 
VON  KRIES  :   Beitriige  zur  Lehre  vom  Augenmass,  Beitrage  zur  Psy- 

chologie  und  Physiologie  der  Sinnesorgane  (Helmholtz  Festgruss), 

Hamburg  und  Leipzig,  1891,  173-193. 
KUNDT  :    Untersuchungen  tiber  Augenmaass  und  optische  Tauschungen, 

Poggendorp's  Annalen,  CXX.,  1863,  118-158. 
LANGE,  N. :   Beitrage  zur  Theorie  der  sinnlichen  Aufmerksamkeit  und 

der  activen  Apperception,  Wundt's  Philos.  Studien,  IV.,  1888,  405  ff. 
LAQUEUR:      Ueber     pseudentoptische      Gesichtswahrnehmungen,   von 

Graefe's  Archiv,  XXXVI.,  1890,  i.,  62-82.     Contains  historical  refer- 
ences. 
LASKA :  Ueber  einige optische  Urtheilstauschungen,  Du  Bois-Reymond's 

Archiv,  1890,  326-328. 
LE  CONTE:   A.  Sight,  New  York,  1881. 

B.  On   an   Optical   Illusion,  Phil.  Mag.,  Ser.  4,  XLI.,  Jan.- June, 
1871,  266-269. 

C.  On  some  Phenomena  of  Binocular  Vision,  No.  XII. :  Some  Pecu- 
liarities of  the  Phantom  Images  formed  by  Binocular  Combination 
of  Regular  Figures,  American  Journal  of  Science,  Series  3,  XXXIV., 
1887,  97-107. 

LIPPS:  A.  Ueber  eine  falsche  Nachbildlokalisation  und  damit  Zusam- 
menhangendes,  Zeitschrift  fiir  Psychologic,  I.,  1890,60-74;  III., 
1892,  493-498.  See  also  ibid.,  II.,  1891,  164-179  (especially  1(>4-1<>7), 
and  III.,  1892,  398-404. 


VISUAL   PERCEPTION  OF  SPACE,    ETC.         329 

B.  Aesthetische  Faktoren  der  Raumanschauung,  Beitrage  zur  Psy- 
chologie  und  Physiologie  der  Sinnesorgane  (Helmholtz  Festgruss), 
Hamburg  und  Leipzig,  1891,  219-307.  For  an  abstract  of  this  paper 
by  Lipps  himself,  see  Zeit.f.  Psy.,  III.,  1892,  219-221. 
D.  Optische  Streitfragen,  Zeitschrift  fur  Psychologic,  III.,  1892, 
493-504. 

LOEB:  A.  Ueber  die  optische  Inversion  ebsner  Linearzeichnungen  bei 
einaugiger  Betrachtung,  Pfliiger's  Archiv,  XL.,  1887,  274-282. 

B.  Untersuchungen  iiber  die  Orientirung  im  Fiihlraum  der  Hand 
und  im  Blickraum,  ibid.,  XLVL,  1890,  1-46. 

C.  Ueber  den  Nachweis  von  Contrasterscheinungen  im  Gebiete  der 
Raumempfindungen  des  Auges,  ibid.,  LX.,  1895,  509-518. 

MACH:  A.  Beitrage  zur  Analyse  der  Empfindungen,  Jena,  1886,  8vo, 
pp.  vi.,  168.  Also  in  English,  translation  by  C.  M.  Williams,  Chi- 
cago, 1897. 

B.  Ueber  die  Wirkung  der  raumlichen  Vertheilung  des  Lichtreizes 
auf  die  Netzhaut,  Sitz.-ber.  d.  k.  Akademie  d.  Wiss.  i.  Wien,  math.- 
nat.  Classe,  LII.,  ii.  Abth.,  1865,  303-322  ;  LIV.,  ii.  Abth.,  1866,  131- 
144;  ibid.,  393-408;  LVII.,  1868,  11-19. 

C.  Beobachtungen  iiber  monoculare  Stereoskopie,  ibid.,  LVIII.,  ii. 
Abth.,  1868,  731-736. 

D.  Ueber  das  Sehen  von  Lagen  und  Winkeln  durch  die  Bewegung 
des  Auges,  ibid.,  XLIII.,  ii.  Abth.,  1861,  215-224. 

MARTIUS:  Ueber  die  scheinbare  Grosse  der  Gegenstande  und  ihre  Be- 
ziehung  zur  Grosse  der  Netzhaut  bilder,  Wundfs  Philos.  Studien, 
V.,  1889,  601-617. 

MARTIUS-MATZDORFF  :  Die  interessantesten  Erscheinungen  der  Stereo- 
skopie, 2d  edit.,  Berlin,  1889.  Thirty-six  diagrams,  with  explana- 
tory pamphlet  of  thirty-five  pages. 

MAYERHAUSEN:  Studies  on  Chromatokinopsias,  Archives  of  Ophthal- 
mology, XIV.,  1885,  81-90. 

MESSER  :  Notiz  iiber  die  Vergleichung  von  Distanzen  nach  dem  Augen- 
maass,  Poggendorff's  Annalen,  CLVII.,  1876,  172-175. 

MULLER-LYER:  A.  Optische  Urtheilstauschungen,  DuBois-Reymond's 
Archiv,  1889,  Supplement-Band,  263-270. 

B.  Zur  Lehre  von  den  optischen  Tauschungen ;  Ueber  Kontrast  und 
Konfluxion,  Zeitschrift  fur  Psychologic,  IX.,  1895,  1-16.  Reply  to 
Laska,  Brentano,  Lipps,  Wundt,  and  Delboeuf. 

(7.  Ueber  Kontrast  und  Konfluxion  (ZweiterArtikel),  ibid.,  X.,  1896, 
421-431.  Reply  to  Heymans. 

MUNSTERBERG:  Augenmass,  Beitrage  zur  experimentellen  Psychologic, 
Heft  2,  1889,  125-181. 

MUNSTERBERG  AND  CAMPBELL  :  The  Motor  Power  of  Ideas,  Psychologi- 
cal Review,  I.,  1894,  441-453. 


330       LABORATORY  COURSE  IN  PSYCHOLOGY. 

NICHOLS  :  Certain  Illusions  of  Rotation,  Proceedings  of  the  American 
Psychological  Association,  pp.  8-9.  (First  Annual  Meeting.  Phil- 
adelphia, 1892.  Pub.  by  Macmillan  Co.,  N.Y.) 

OPPEL:  B,  Ueber  ein  Anaglyptoskop  (Vorrichtung,  vertiefte  Formeii 
erhaben  zu  sehen),  Poggendorjf's  Annalen,  XCIX.,  1856,  466-469. 

PIERCE  :  ^Esthetics  of  Simple  Forms,  (1)  Symmetry,  Psychological  He- 
view,  I.,  1894,  483-495. 

QUANTZ  :  The  Influences  of  the  Color  of  Surfaces  on  our  Estimation  of 
their  Magnitude,  American  Journal  of  Psychology,  VII.,  1895-96, 
26-41. 

RIVERS:   On  the  Apparent  Size  of  Objects,  Mind,  N.S.  V.,  1896,  71-80. 

ROGERS:  A.  Observations  on  Binocular  Vision,  American  Journal  of 
Science,  Series  2,  XX.,  1855,  86-98,  204-220,  318-335  ;  XXI.,  1856, 
80-95,  173-189,  439  (errata,  p.  viii). 

B.  Some  Experiments  and  Inferences  in  Regard  to  Binocular  Vision, 
ibid.,  Series  2,  XXX.,  1860,  387-390;  or  Proc.  Amer.  Assoc.,  1860, 
187-192. 

C.  On  our  Inability  from  the  Retinal  Impression  alone  to  determine 
which  Retina  is  Impressed,  Proc.  Amer.  Assoc.,  1860,  192-198;  or 
Amer.  Jour.  Science,  Ser.  2,  XXX.,  1860,  404-409. 

ROOD  :  On  the  Relation  between  our  Perception  of  Distance  and  Color, 
American  Journal  of  Science,  Ser.  2,  XXXII.,  1861,  184-185. 

ROUSE:  The  Visual  Perception  of  Distance,  Kansas  University  Quar- 
terly, V.,  1896,  109-117. 

SCHAPRINGER:  Zur  Theorie  der  "  Flatternden  Herzen,"  Zeitschrift  fii. • 
Psychologic,  V.,  1893,  385-396. 

SCHARWIN  UND  NovizKi  :  Ueber  den  scheinbaren  Grossen  wechsel  der 
Nachbilder  im  Auge,  Zeitschrift  fur  Psychologic,  XL,  1896,  408-409. 

SCHON:  A.  Zur  Lehre  vom  binocularen  indirecten  Sehen,  von  Graefe's 
Archiv,  XXII.,  1876,  iv.,  31-62. 

B.  Zur  Lehre  vom  binocularen  Sehen,  II.  Aufsatz,  ibid.,  XXIV., 
1878,  i.,  27-130. 

C.  Zur  Lehre  vom  binocularen  Sehen,  III.  Aufsatz,  ibid.,  iv.  47- 
116. 

SORET  :   Des  Conditions  physiques  de  la  Perception  du  Beau,  Geneve, 

1892. 
STERN:    Die  Wahrnehmung  von  Bewegungen  vermittelst  des  Auges, 

Zeitschrift  fur  Psychologic,  VII.,  1894,  321-386. 
STEVENS:    A.  The  Stereoscope  and  Vision  by  Optic  Divergence,  Amer. 

Jour.  Science,  Series  3,  XXII. ,  1881,  358-362,  443-451. 

B.  Notes  on  Physiological  Optics,  ibid.,  Series  3,  XXIII.,  1882,  290- 

302,  346-300;  XXIV.,  1882,  241-247,  331-335. 


VISUAL  PERCEPTION  OF  SPACE.  331 

(7.  Physiological  Perspective,  Phil.  Mag.,  Ser.  5,  XIII.,  Jan.-June, 
1882,  309-322. 

STRATTON  :  Some  Preliminary  Experiments  on  Vision  without  Inversion 
of  the  Ketinal  Image,  Psychological  Review,  III.,  1896,  611-617.  See 
also  further  discussion  by  Hyslop  and  others  in  Vol.  IV.  of  the  same 
journal,  and  Science  N.S.  II. 

SULLY  :  Illusions,  New  York,  1882. 

SZILI:  A.  Zur  Erklarung  der  "Flatternden  Herzen,"  Du  Bois-Rey- 
mond's  Archiv,  1891,  157-163. 

B.  "Flatternde  Herzen,"  Zeitschrift  fur  Psychologic,  III.,  1891-92, 
359-387. 

THIERY:  Ueber  geometrisch-optische  Tauschungen,  Wundt's  Philos. 
Studien,  XI.,  1895,  307-370,  603-620;  XII.,  1896,  67-126. 

THOMPSON,  S.  P. :  A.  On  the  Chromatic  Aberration  of  the  Eye  in  rela- 
tion to  the  Perception  of  Distance,  Phil.  Mag.,  Ser.  5,  IV.,  July- 
Dec.,  1877,  48-60. 

B.  Optical  Illusions  of  Motion,  Brain,  III.,  1880-81,  269-298;  also 
Popular  Science  Monthly,  XVIII.,  1880-81,  519-526. 

WALLENBERG:  Der"Le  Cat'sche  Versuch"  und  die  Erzeugung  far- 
biger  Schatten  auf  der  Netzhaut,  Pfliiger's  Archiv,  XLVIII.,  1890- 
91,  537-543. 

WALLER:   A  New  Colour-contrast  Experiment,  Journal  of  Physiology, 

XII.,  1891,  xliv.-xlix.     (Proceedings  of  the  Physiological  Society, 

June  20,  1891.) 
WARREN  AND  SHAW  :  Further  Experiments  on  Memory  for  Square  Size, 

Psychological  Review,  II.,  1895,  239-244. 
WASHBURN,  MARGARET  F. :   The  Perception  of  Distance  in  the  Inverted 

Landscape,  Mind,  N.S.  III.,  1894,  438-440. 

WHEATSTONE:  A.  Contributions  to  the  Physiology  of  Vision,  Part  I. 
On  Some  Remarkable  and  hitherto  unobserved  Phenomena  of  Binoc- 
ular Vision,  Philosophical  Transactions,  1838,  pt.  ii.,  371-394.  Also 
Fogg.  Ann.  (Ergdnz.  Bd.  I.),  1842, 1-48,  and  Phil.  Mag.,  Ser.  4.  III., 
Jan.-June,  1852,  241-267. 

B.  Contributions  to  the  Physiology  of  Vision,  Part  II.,   On  Some 
Remarkable  and  hitherto  Unobserved  Phenomena  of  Binocular  Vis- 
ion, Philosophical  Transactions,  1852,  1-17;  Phil.  Mag.,  Ser.  4,  III., 
Jan.-June,  1852,  504-523. 

C.  On  a  Singular  Effect  of  the  Juxtaposition  of  Certain  Colours 
under  Particular  Circumstances,  with  remarks  by  Sir  D.  Brewster, 
Brit.  Assoc.  Rep.,  1844  (Pt.  2),  10. 

WOOD:  The  "Haunted  Swing"  Illusion,  Psychological  Review,  II., 
1895,  277. 


332      LABORATORY  COURSE  IN  PSYCHOLOGY. 

WUNDT:  A.  Grundziige  der  physiologischen  Psychologie,  4te  Aufl., 
Leipzig,  1893. 

B.   Beitrage   zur  Theorie  der  Sinneswahrnehmung,   Leipzig    und 
Heidelberg,  1862. 

ZOLLNER:  A.  Ueber  eine  neue  Art  von  Pseudoskopie  und  ilire  Bezie- 
hungen  zu  den  von  Plateau  und  Oppel  beschriebenen  Bewegungs- 
phanomenen,  Poggendorjf's  Annalen,  CX.,  1860,  500-523. 

B.  Ueber   die   Abhangigkeit    der    pseudoskopischen  Ablenkung 
paralleler  Linien  von  dem  Neigungswinkel  der  sie  durchschneiden- 
den  Querlinien,  ibid.,  CXIV.,  1861,  587-591. 

C.  Ueber  eine  neue  Art  anorthoskopischer  Zerrbilder,  ibid.,  CXVII., 
1862, 


WEBER'S  LAW.  333 


CHAPTER   VIII. 
Weber's  Law  and  the  Fsychophysic  Methods. 

THE  purpose  of  this  chapter  is  not  an  extended  treat- 
ment of  these  most  technical  and  still  debated  matters,  but 
simply  the  bringing  together  of  a  few  demonstrations  of 
the  general  nature  of  Weber's  law,  and  the  suggestion  of  a 
few  practice  experiments  for  the  psychophysic  methods, 
with  their  necessary  precautions,  and  examples  of  the 
treatment  of  the  results  obtained.  For  this  reason  refer- 
ences to  literature  need  not  go  beyond  the  text-books 
(Wundt  and  Kiilpe,  for  example),  and  a  few  easily  accessi- 
ble special  articles.  Should  fuller  information  be  desired, 
the  student  will  not  fail  to  go  to  original  sources  in  Fech- 
ner,  G.  E.  Miiller,  and  later  articles  by  several  hands  in 
Wuiidt's  Philosophische  Studien. 

WEBER'S  LAW. 

Weber's  original  discovery  was  that  ability  to  distin- 
guish stimuli  depends,  not  on  their  absolute,  but  on  their 
relative,  difference.  If,  for  example,  the  pressure  of  four 
ounces  on  one  hand  could  just  be  distinguished  from  that 
of  three  ounces  on  tfye  other,  the  pressure  of  four  pounds 
would  be  just  distinguishable  from  that  of  three  pounds,  and 
the  same  with  other  weights  in  the  same  ratio.  It  has 
also  been  found  that  when  the  question  is  not  one  of  just 
observable  differences,  but  of  those  much  larger,  the  same 
principle  of  relativity  holds.  If,  for  example,  a  series  of 
lights  must  be  adjusted  in  such  a  way  that  the  increase 


334       LABORATORY  COURSE  IN  PSYCHOLOGY.    [234 

of  brightness  from  light  to  light  shall  seem  equal  in  each 
case,  it  will  be  necessary  to  multiply  the  intensity  each 
time  by  a  constant  factor,  not  to  add  to  it  each  time  a  fixed 
amount.  In  general,  in  order  to  increase  sensation  by 
equal  increments,  it  is  necessary  to  increase  the  stimulus 
by  proportional  increments ;  or,  in  other  words,  to  increase 
sensation  in  arithmetical  ratio,  it  is  necessary  to  increase 
the  stimulus  in  geometrical  ratio. 

The  law  thus  formulated  is  an  empiric-al  law  unifying  a 
considerable  number  of  facts.  It  holds  tolerably  for  me- 
dium stimuli  of  several  kinds.  With  very  large  and  very 
small  stimuli  it  holds  imperfectly,  and  with  some  stimuli  it 
cannot  be  demonstrated. 

The  general  nature  of  the  law  is  shown  most  easily  in  the 
case  of  visual  intensities. 

234.    Demonstrational  Experiments  for  Weber's  Law. 

a.  Transparencies.  Provide  a  photographic  transpar- 
ency of  a  scene  or  object  presenting  a  few  very  faint  shad- 
ows or  slight  differences  in  shading.  (One  in  the  Clark 
laboratory  shows  a  gray  stone  church  with  a  number  of 
slight  discolorations  on  the  roof.)  The  perception  of  these 
depends  on  the  perception  of  the  difference  of  intensity 
in  the  light  coming  through  them  and  through  adjacent 
parts  of  the  transparency.  To  make  a  test  of  the  law, 
therefore,  it  is  only  necessary  to  examine  them  under 
varied  conditions  of  illumination.  Try,  for  example,  when 
the  transparency  is  before  a  shaded  white  wall,  before  the 
same  diffusely  lighted,  and  before  the  same  in  strong  light, 
taking  care  to  avoid,  as  far  as  possible,  reflections  from  the 
front  surface  of  the  glass.  The  shadows  will  be  found  about 
equally  distinct  in  all  cases,  after  the  eye  has  become  ac- 
commodated to  the  change.  The  reflection  can  be  avoided 
in  part  by  looking  at  the  transparency  through  a  half-inch 
hole  in  a  large  piece  of  black  cardboard.  A  variable  back- 


234]  WEBEirS   LAW.  335 

ground  can  also  be  made  by  placing  a  large  piece  of  white 
cardboard  in  the  sunlight  and  changing  its  inclination. 

Try  also  against  a  bit  of  clear  sky  near  the  sun.  The 
finer  shadows  will  be  seen  less  well,  or  may  entirely  disap- 
pear, showing  the  failure  of  the  law  with  stimuli  of  great 
intensity.  Try  also  with  full  sunlight. 

A  similar  test  may  be  made  by  repeating  Ex.  140  b  under 
varying  conditions  of  illumination. 

b.  Demonstration  with  Disks.  It  is  not  difficult  to  pro- 
vide a  gradation  of  intensity  with  rotating  disks.  On  a 
disk  constructed  to  give  a  geometrical  increase  of  intensity 
from  centre  to  circumference,  the  medium  gray  ought,  if 
Weber's  law  holds,  to  stand  half-way  out  from  the  centre ; 
and  such  is  the  case.  The  following  figures  represent  disks 
of  this  kind  constructed  by  Kirschmann. 


B 


A  gives  a  gradual  increase  in  intensity  from  centre  to 
circumference.  B  shows  a  gradual  decrease.  It  is  con- 
venient to  have  also  for  comparison  a  disk  in  which  the 
change  is  in  arithmetical  ratio.  Such  a  disk  is  shown  in 
C  below.  D  gives  the  same  geometrical  gradation  as  A, 


336       LABORATORY   COURSE  IN  PSYCHOLOGY.     [235 

but  with  a  different  arrangement  of  the  black  on  the  disk ; 
the  result  in  both  is  the  same. 

Helmholtz,   G.   384  ff.,  Fr.  411   ff.   (309  ff.);  Delboeuf,  91  ff.; 
Kirschmann. 


235.  Irradiation.  The  form  of  this  phenomenon,  which 
consists  in  the  enlargement  of  bright  areas  at  the  expense 
of  adjacent  dark  ones,  is  known  as  Positive  Irradiation, 
and,  as  usually  explained,  furnishes  an  interesting  illustra- 
tion of  Weber's  law.  An  enlargement  of  lines  and  points 
at  the  expense  of  adjacent  areas,  either  light  or  dark,  some- 
times called  Negative  Irradiation,  though  wholly  unrelated 
to  Weber's  law,  will  also  be  considered  in  this  connection. 

a.  Positive  Irradiation.  Diagrams  like  that  on  the  op- 
posite page,  when  strongly  illuminated  and  viewed  from  a 
little  distance,  show  the  white  square  slightly  larger  than 
the  black,  though  both  are  actually  of  a  size. 

The  enlargement  is  unmistakable  when  a  diagram  pre- 
senting greater  differences  of  illumination  between  the 
light  and  dark  parts  is  secured  by  cutting  the  latter  from 


235] 


WEBER'S  LAW. 


337 


black  cardboard,  mounting  them  on  glass,  and  viewing  the 
whole  against  the  sky  or  a  brightly  lighted  surface.  The 
effect  is  also  very  marked  when  a  ruler  or  straight-edged 


piece  of  cardboard  is  held  before  a  flame.  The  flame  seems 
to  cut  into  the  edge,  —  the  brightest  parts  most.  When  the 
new  moon  "  holds  the  old  moon  in  its  arms,"  the  crescent 
seems  to  belong  to  a  larger  disk  than  that  between  its 
tips,  and  many  other  illustrations  may  be  met  in  common 
experience. 

The  illusion,  ar  Helmholtz  explains  it,  depends  on  a 
slight  blurring  of  the  line  of  demarcation  between  the 
black  and  white  areas  —  a  blurring  which  is  very  marked 
when  the  illumination  is  strong  and  accommodation  is  in- 
exact, and  is  not  entirely  absent  even  when  accommodation 
is  at  its  best.  The  contour  is  thus  spread  out  into  a  nar- 
row gray  band,  lying  partly  in  the  black  and  partly  in  the 
white,  and  shading  over  from  one  to  the  other.  The  in- 
tensities of  light  in  different  parts  of  this  band  are  repre- 
sented in  the  curve  shown  on  the  next  page.  The  point 
c  is  taken  on  the  actual  line  of  demarcation ;  the  region  to 
the  left  represents  the  white  area,  that  to  the  right  the 
black. 

If  there  were  no  blurring,  the  white  would  extend  up  to 
c  d,  and  there  cease,  and  the  intensity  curve  would  be  ad  eg. 


838       LABORATORY   COURSE  IN  PSYCHOLOGY.      [235 

With  the  blurring,  we  have  full  white  to  the  left  of  a,  and 
full  black  to  the  right  of  g,  and  the  gray  band  of  transition 
between,  with  intensities  represented  by  the  curve  afg. 
This  curve,  however,  shows  intensity  of  light,  and  not  in- 
tensity of  sensation.  According  to  Weber's  law,  the  just 
observable  change  in  sensation  would  require  a  more  con- 
siderable change  in  stimulus  at  the  intense  end  of  the  curve 
than  anywhere  else,  and  any  less  change  would  pass  en- 
tirely unperceived.  If  the  white  were  very  intense,  —  so 
intense  that  a  practical  maximum  of  sensation  were  reached 
with  an  intensity  h,  —  no  difference  could  be  perceived  to 
the  left  of  hj  and  the  white  would  seem  to  extend  in  full 
intensity  to  that  point,  enlarging  its  area  by  eh.  If  the 
middle  gray  of  the  band,  instead  of  the  white  edge,  were 


taken  as  the  boundary,  that  also  would  be  found  to  lie  too 
far  to  the  right,  and  would  enlarge  the  white  area.  The  im- 
portance of  high  illumination  hi  the  white,  and  complete 
blackness  in  the  black,  is  evident  in  either  case.  The  illu- 
sion would  admit  the  same  explanation  whether  the  blur- 
ring were  due  to  a  physical  dispersion  of  the  light  on  the 
retina,  or  to  a  physiological  spread  in  the  nervous  elements. 
b.  Negative  Irradiation.  In  this  case  the  line  or  point 
is  given  the  benefit  of  practically  all  of  its  gray  border, 
near  the  outer  edge  of  which  there  is,  according  to  Helm- 


235]  WEBER'S  LAW.  339 

holtz,  a  relatively  sudden  increase  of  brightness  (or  de- 
crease in  the  case  of  a  white  line  on  a  dark  ground)  which 
fixes  the  apparent  boundary.  Because  of  this  difference  in 
the  nature  of  the  phenomenon,  Helmholtz  would  prefer  not 
to  call  this  irradiation  at  all. 

The  combined  action  of  positive  and  negative  irradiation 
seems  to  account  for  the  tendency  of  the  white  circles  in. 
the  following  diagram  to  take  an  hexagonal  shape  when 
viewed  from  distances  that  make  accommodation  inexact. 
The  larger  triangular  area?  suffer  by  positive  irradiation 
of  the  circles,  while  the  portions  where  the  circles  approach 


nearest  each  other  owe  their  preservation  to  negative  irra- 
diation. 

Negative  irradiation  may  also  be  demonstrated  as  fol- 
lows. On  smooth  white  paper  draw  two  fine  black  lines 
(.25  mm.  or  less  in  breadth),  intersecting  at  an  angle  of 
one  or  two  degrees.  Hold  the  paper  at  arm's  length,  and 
determine  by  eye  the  place  where  the  white  space  between 
the  lines  is  equal  in  breadth  to  one  of  the  lines.  Mark 


340       LABORATORY   COURSE  IN  PSYCHOLOGY.      [236 

the  point  lightly  with  a  pencil,  and  examine  it  under  a  lens. 
It  will  be  found  that  the  separation  at  the  point  selected 
is  too-  great,  the  breadth  of  the  lines  having  been  overesti- 
mated. 

Helmholtz,  G.  394-402,  Fr.  425-433  (321-327);  Hering  ;  Aubert, 
575  ff.,  581  ff.  All  of  these  refer  to  further  literature. 

236.  Weber's  Law  in  the  Classification  of  Stimuli.  It 
has  been  found  that  when  a  large  number  of  slightly  differ- 
ent stimuli  are  arranged  in  groups  that  seem  to  form  a 
series  with  equal  differences,  the  average  intensities  of  the 
groups  conform  more  or  less  exactly  to  Weber's  law.  This 
has  been  proved  to  be  the  fact  on  a  magnificent  scale  in 
the  grouping  of  the  stars  in  magnitudes,  and  a  rough 
demonstration  by  a  similar  method  is  not  difficult  to  make 
in  the  laboratory. 

Prepare  a  large  number  of  weights,  each  differing  slightly 
from  the  other,  by  enclosing  pieces  of  sheet-lead  in  stout 
envelopes.  (Cf.  Chapter  IX.  for  further  details.)  Select 
the  lightest  and  the  heaviest  of  the  lot,  and  give  them  to 
the  subject  as  standards  from  which  he  may  from  time  to 
time  refresh  his  memory  of  the  range  of  the  series.  Then 
require  him  to  arrange  the  rest  in  five  classes,  about  equally 
separate  in  the  intensity  scale.  When  all  have  been  classed, 
allow  the  subject  to  go  over  each  group,  and  revise  his 
rating  if  he  desires.  Finally,  find  the  average  weight  in 
each  group  (by  weighing  the  whole  group  at  once  and  di- 
viding by  the  number  of  envelopes  in  it),  and  calculate  the 
ratios  of  these  from  group  to  group. 

The  following  records  of  a  trial  with  four  subjects  may 
be  interesting  for  comparison.  A  series  of  118  envelopes, 
ranging  approximately  from  5  to  100  grams,  was  sorted. 
In  the  table  below,  the  groups  are  indicated  by  Roman 
numerals,  beginning  with  the  lightest. 


236] 


THE  PSYCUOPHYSIC  METHODS. 


341 


SUBJECTS. 

Ratio  of 
11:  1. 

Ratio  of 
III  :  11. 

Ratio  of 
IV:  III. 

Ratio  of 
V:IV. 

Sh. 

1.85 

1.86 

1.77 

1.77 

St. 

1.51 

1.90 

1.62 

2.16 

D. 

2.39 

1.08 

1.58 

1.52 

C. 

1.34 

1.84 

1.73 

1.96 

The  first  shows  a  fair  approximation  to  the  constancy  of 
ratio  required  by  the  law.  The  third  and  fourth  give  some 
evidence  of  it  from  the  second  group  on. 

This  method  of  demonstration  is  a  special  application 
of  the  Method  of  Average  Error  to  difference  comparison. 
The  general  method  is  considered  more  fully  below.  Clas- 
sification has  been  tried  by  Jastrow  with  extents,  both 
visual  and  kinsesthetic,  and  time  intervals,  and  by  Leuba 
on  artificial  stars. 

Jastrow,  B  and  C  ;  Leuba. 

THE  PSYCHOPHYSIC  METHODS. 

In  the  quantitative  study  of  sensations  and  stimuli,  four 
questions  arise :  (1)  What  is  the  least  amount  of  a  given 
stimulus  that  will  cause  a  sensation  at  all  (the  "  initial 
threshold ")  ?  (2)  What  amount  of  stimulus  applied  to 
one  region  of  the  body,  or  under  one  set  of  circumstances, 
seems  exactly  equal  to  a  given  amount  applied  elsewhere, 
or  under  other  circumstances  (equivalent  stimuli)  ?  (3) 
What  is  the  least  difference  that  can  be  perceived  between 
two  given  stimuli  (the  "  differential  threshold  ")  ?  (4) 
What  is  the  relation  of  several  stimuli  when  their  differ- 
ences among  themselves  seem  equal  ? 1 


1  In  Kiilpe's  terms  these  questions  are  those  of  (1)  Stimulus  determination, 
(2)  Stimulus  comparison,  (3)  Difference  determination,  and  (4)  Difference  com- 
parison. 


342       LABORATORY  COURSE  IN  PSYCHOLOGY. 

It  is  in  attempting  to  answer  such  questions  as  these  that 
the  psychophysic  methods  have  been  developed.  They  are 
three  in  number  :  The  Method  of  Minimal  Change,  the 
Method  of  Eight  and  Wrong  Cases,  and  the  Method  of 
Average  Error.  They  have  been  most  frequently  applied 
in  determinations  of  the  least  observable  difference  of  stim- 
uli (question  3,  above),  and  in  that  connection  can  be 
readily  made  clear. 

The  most  natural  way  of  finding  such  a  difference  is  to 
begin  with  the  stimuli  equal,  and  very  gradually  change 
one  of  them  till  the  fact  of  change  is  just  apparent.  This 
is  the  Method  of  Minimal  Change,  —  the  method  used  in 
a  number  of  experiments  in  earlier  chapters,  and  explained 
somewhat  fully  in  Ex.  24. 

An  indication  of  the  amount  of  the  just  observable  dif- 
ference may  also  be  found,  but  less  directly,  if  the  two 
stimuli  are  made  a  little  different,  and  the  subject  is  re- 
quired to  judge  of  their  relative  condition  a  large  number 
of  times.  If  they  are  quite  different  he  will  perceive  the 
difference  with  a  good  deal  of  certainty,  and  judge  right 
nearly  every  time ;  if  they  are  very  little  different,  he  will 
be  guided  largely  by  chance,  and  judge  wrong  nearly  as 
often  as  right.  The  proportion  of  right  judgments  in  a 
long  series  of  trials  taken  in  connection  with  the  amount 
of  difference  between  the  stimuli  used,  makes  inference 
possible  with  regard  to  the  just  observable  difference. 
This  is  the  Method  of  Eight  and  Wrong  Cases. 

The  third  method,  like  the  last,  is  indirect,  and  gives, 
instead  of  the  just  observable  difference,  a  quantity  bear- 
ing a  more  or  less  constant  relation  to  it.  One  of  the 
stimuli  is  put  under  the  control  of  the  subject,  and  he  is 
required  to  adjust  it  to  equality  with  the  other.  His 
adjustments  will  often  be  slightly  in  error,  sometimes  by 
excess  and  sometimes  by  defect.  The  range  of  these  clevi- 


THE  PSYCHOPHYSIC  METHODS.  343 

ations  will  evidently  bear  some  relation  to  the  least  differ- 
ence that  he  can  perceive ;  and  on  this  fact  is  based  the 
Method  of  Average  Error. 

The  methods  thus  rudely  sketched  have  received  con- 
siderable elaboration  in  actual  use,  and  certain  precautions 
have  been  found  necessary,  the  most  important  of  which 
will  be  noticed  below. 

In  theory,  any  of  the  methods  might  be  used  for  answer- 
ing all  four  questions.  As  a  matter  of  fact,  however,  some 
are  much  better  adapted  for  certain  purposes  than  others, 
and  all  are  more  or  less  modified  by  the  special  conditions 
under  which  they  are  used.  The  Method  of  Minimal 
Change  has  received  a  variety  of  names  in  its  special  ap- 
plications which,  unless  noticed,  may  lead  to  confusion. 
In  determinations  of  stimuli  that  appear  equal  under  dif- 
ferent circumstances,  it  has  been  called  the  "Method  of 
Equivalents."  In  determinations  of  the  least  observable 
difference  of  stimuli  it  has  been  called  the  "Method  of 
Just  Observable  Difference,"  or  the  "  Method  of  Minimal 
Change,"  in  a  restricted  sense.  In  determinations  of 
equal  differences  it  has  been  called  the  "  Method  of  Mean 
Gradation." 

Though  developed  in  the  first  instance  for  the  study  of 
Weber's  law,  these  methods  have  a  much  wider  useful- 
ness as  tests  of  the  keenness  and  correctness  of  perception 
under  differing  external  and  internal  conditions,  and  are  in 
constant  use  for  that  purpose. 

All  the  methods  apply  to  both  extensive  and  intensive 
magnitudes.  The  most  convenient  stimuli  for  demonstra- 
tional  experiments  will  be  found  in  visual  extents  and 
in  weights,  used  either  as  lifted  weights  or  for  pressure. 
Experiments  with  the  first  have  the  great  advantage  of 
seeming  less  irksome  and  monotonous;  and,  if  necessary, 
all  three  methods  can  be  shown  with  visual  extents  alone. 


344       LABORATORY  COURSE  IN  PSYCHOLOGY.      [237 

In  the  fuller  explanations  that  follow,  the  methods  are 
again  considered  in  their  application  to  difference  deter- 
mination. 

237.  The  Method  of  Minimal  Change.  If  the  student 
is  not  already  familiar  with  this  method  from  its  applica- 
tion in  earlier  chapters,  it  can  be  tried  with  pressures,  as 
in  Ex.  24  (if  possible  with  the  pressure  balance,  and  with 
attention  to  the  precautions  mentioned  below),  or  with 
visual  extents,  as  in  Ex.  174  b  (where  the  krypteon  may  be 
dispensed  with),  or  indeed  with  almost  any  of  the  experi- 
ments given  for  discriminative  sensibility. 

Whatever  the  stimuli  selected,  the  procedure  falls  into 
four  stages:  (1)  The  determination  of  the  stimulus  just 
observably  greater  than  the  standard  stimulus ;  (2)  the  de- 
termination of  that  just  ^observably  greater ;  (3)  the  deter- 
mination of  that  just  observably  less ;  and  (4)  the  determi- 
nation of  that  just  ?mobservably  less. 

In  the  first  stage,  a  standard  stimulus  is  applied,  and 
immediately  after  it  another  exactly  like  it  (or  at  least  not 
perceptibly  different  from  it).  The  subject  reports  them 
"  the  same.7'  The  standard  is  again  applied,  and  after  it  a 
slightly  greater  stimulus,  which  the  subject  usually  calls 
"  the  same  "  again.  Comparison  of  the  standard  stimulus 
with  successively  greater  stimuli  is  thus  continued  till  one 
is  found  which  the  subject  reports  as  just  noticeably 
greater.  The  excess  of  the  variable  stimulus  over  the 
standard  is  then  recorded.  The  first  stage  might  end  here, 
but  it  is  considered  better  to  increase  the  variable  stimulus 
once  or  twice  more  in  order  to  guard  the  subject  against  a 
merely  accidental  impression  that  a  perceptible  difference 
has  been  reached,  though  no  record  is  made  of  the  differ- 
ences used  unless  it  is  evident  that  his  previous  success  was 
accidental.  The  second  stage  is  like  the  first,  except  that 
it  begins  with  the  comparison  of  the  standard  with  a  vari- 


237] 


THE  PSYCHOPHYSIC  METHODS. 


345 


able  stimulus  unmistakably  greater,  and  that  the  latter  is 
reduced  little  by  little  till  it  just  ceases  to  seem  different. 
The  excess  of  the  variable  at  this  point  is  recorded,  and 
one  or  two  confirmatory  trials  made  as  before.  The  third 
stage  is  like  the  first,  except  that  the  variable  stimulus  is 
gradually  decreased  from  apparent  equality,  and  the  fourth 
like  the  second,  except  that  the  trials  begin  with  the  vari- 
able stimulus  unmistakably  less  than  the  standard.1 

The  records  reached  in  the  four  stages  are  subject,  of 
course,  to  more  or  less  of  accidental  variation,  to  avoid 
which  it  is  necessary  to  repeat  each  a  number  of  times,  and 
to  average  the  results.  Care  must  also  be  taken  to  avoid 
certain  other  sources  of  error,  which  will  be  considered 
after  the  arithmetical  treatment  of  the  results  has  been 
made  clear  by  an  example. 

Let  us  suppose  that  in  an  experiment  with  pressures  on 
the  finger-tip  the  standard  stimulus  has  been  25  grams, 
and  that  the  following  differences  have  been  found  in  the 
ten  trials  indicated  by  the  Roman  numerals  2 :  — 


DIKECTIOX  OF  CHANGE. 

i 

ii 

in 

IV 

V 

VI 

VII 

VIII 

IX 

X 

From  equality  upward       .     . 

1.0 

0.8 

0.6 

1.2 

0.8 

0.6 

1.0 

1.2 

0.6 

0.6 

Toward  equality  downward  . 

1.0 

0.8 

1.4 

1.2 

1.0 

1.2 

1.4 

1.0 

0.6 

1.0 

From  equality  downward 

1.4 

1.0 

1.6 

1.4 

1.2 

1.2 

1.4 

1.4 

1.4 

1.0 

Toward  equality  upward  . 

2.0 

1.2 

1.8 

2:0 

2.0 

1.8 

1.8 

1.4 

1.8 

0.6 

If  we  take  the  averages  of  the  ten  series  for  each  of  the 

four  stages  we  shall  have  the  following  values  in  grams  for 


1  The  just  observable  and  just  unobservable  differences  are  combined  in  both 
cases  on  the  supposition  that  the  subject  will  tend  to  report  the  expected  result 
too  soon  (or  too  late),  and  that  the  true  just  observable  difference  will  lie 
between  them. 

2  The  figures  of  this  example  are  but  slightly  changed  from  those  of  an  actual 
experiment  with  the  pressure  balance. 


346       LABORATORY  COURSE  IN  PSYCHOLOGY.     [237 

the  just  observable  difference  or  limen,  with  the  accompany- 
ing mean  variations.1 

LIMEN.  M.Y. 

Changing  from  equality  (above)  0.84  0.21 

"         toward      "         (above)  1.06  0.19 

"  from       "        (below)  1.30  0.16 

"         toward      "         (below)  1.64  0.24 

Examination  of  these  figures  shows  first  that  the  mean 
variations  are  large  in  proportion  to  the  averages,  and  there- 
fore that  the  determinations  are  irregular,  and  in  so  far 
uncertain.  Disregarding  that  in  this  instance,  however,  let 
us  examine  the  averages  themselves.  Averaging  the  four 
determinations,  we  have  1.21  as  the  average  just  observable 
difference  (or  average  limen  of  difference),  the  reciprocal  of 
which  is  taken  as  the  measure  of  the  keenness  of  the  dis- 
crimination of  the  subject  under  experiment.  Stated  as  a 

1  The  mean  variation  (M.  V.)  is  found  by  substracting  algebraically  from 
the  average  each  of  the  terms  that  has  entered  into  the  average,  and  taking  the 
mean  of  these  remainders  without  reference  to  sign.  Thus  the  terms  of  the  first 
line  across  the  table  subtracted  each  from  0.84  give  the  following  remainders  :  — 
+  0.16,  —0.04,  —0.24  +  0.36,  —0.04,  —0.24  +  0.16  +  0.36,  —0.24,  —0.24,  which  av- 
eraged without  reference  to  sign  gives  0.21  as  the  mean  variation. 

A  convenient  way  of  checking  the  correctness  of  the  average  and  most  of 
the  process  of  finding  the  mean  variation  is  to  keep  the  plus  and  minus  remain- 
ders separate  as  long  as  possible.  If  their  sums  are  the  same  the  work  is  correct 
to  that  point  ;  e.g.  :  — 

+  0.16,  0.36,  0.16,  0.36  =  1.04 

—0.04,  0.24,  0.04,  0.24,  0.24,  0.24=  1.04 
1.04+1.04  =  2.08;  2.08-i-lO     =0.208 
approximately  0.21. 

Expressing  the  rule  in  a  formula  gives  :— 

—  q3)+  •  •  •  (A  —  an) 


in  which  A  is  the  average,  at  a2  a3,  etc.,  are  the  individual  observations,  and  n 
the  number  of  the  observations  entering  the  average.  In  mo're  condensed  form 
the  formula  stands  :  — 


in  which  2  is  the  sign  for  summation,  v  stands  for  the  individual  variations  of 
the  observations  from  the  mean,  and  n  for  the  number  of  observations. 


237]  THE  PSYCHOPHYSIC  METHODS.  347 

ratio  with  the  standard  we  have  1.21 :  25,  or  about  1 :  20  5 
in  per  cent  4.8.  Combining  the  pairs  of  determinations 
above  and  below  the  standard,  we  have  0.95  as  the  just 
observable  increase  of  weight  (or  the  upper  limen  of  dif- 
ference), and  1.47  as  the  just  observable  decrease  (or  lower 
limen  of  difference). 

It  is  clear  that  much  greater  change  was  necessary  below 
the  standard  than  above  it,  though  approximate  equality 
might  have  been  expected.  This  points  to  a  constant  ten- 
dency toward  underestimation  of  the  standard  (or  overesti- 
mation  of  the  variable  stimulus)  —  toward  a  constant  error. 
This  amounts  to  half  the  difference  of  the  limens,  or  0.26, 
making  the  estimated  or  effective  value  of  the  standard  :  — 

25  —  0.26  =  24.74. 

We  now  return  to  some  of  the  conditions  required  in  the 
careful  application  of  the  method.  The  most  important  of 
these,  after  a  uniform  condition  of  attention  on  the  part 
of  the  subject,  is  such  an  arrangement  of  the  tests  as  will 
exclude  or  neutralize  all  recognized  sources  of  constant 
errors.  Two  of  these  are  of  such  frequent  occurrence  as  to 
have  received  special  designations ;  namely,  the  Time  Error 
and  the  Space  Error.  It  is  impossible  to  present  both  the 
standard  and  the  variable  stimulus  to  the  same  sensory  sur- 
face at  the  same  time ;  they  must  be  applied  at  different 
places  or  at  different  times.  If,  therefore,  there  is  any  pe- 
culiarity of  perception  attaching  to  one  place  or  the  other, 
or  to  the  leading  or  following  position  in  time  order,  constant 
differences  will  be  introduced.  In  the  example  above,  the 
space  error  has  been  avoided  by  applying  both  stimuli  to 
the  same  finger-tip,  but  the  time  error  remains  unless  spe- 
cially met.  This  is  done  by  dividing  the  tests,  and  giving 
half  in  the  order  described  and  half  in  the  reverse  order 

the  variable  stimulus  being  applied  first  and  the  standard 


348       LABORATORY  COURSE  IN  PSYCHOLOGY.      [237 

following.  If  this  had  not  been  done  in  the  example  con- 
sidered, the  constant  error  0.26  would  not  be  attributable 
as  suggested,  but  might  probably  indicate  an  unexcluded 
time  error.  In  the  visual  comparison  of  lengths  the  time 
error  is  supposed  to  be  avoided,  but  the  space  error  must 
be  compensated  by  presenting  the  standard  length  at  the 
right  of  the  variable  length  as  often  as  at  the  left,  and  sim- 
ilarly with  tests  in  the  fields  of  other  senses.  Differences 
in  practice  may  sometimes  be  treated  in  a  similar  way,  but 
where  certainty  of  result  is  required,  must  be  excluded  by 
a  preliminary  course  of  training.  To  ascertain  whether 
changes  in  practice  are  introducing  changes  in  the  results, 
the  records  should  be  divided  into  sub-groups,  and  their 
averages  examined. 

The  time  and  space  errors  and  the  stage  of  practice  must 
be  regarded  in  the  use  of  all  the  methods.  In  the  case  of 
the  Method  of  Minimal  Change  the  tests  above  and  below 
the  standard,  both  working  toward  it  and  away  from  it, 
must  be  so  alternated  as  to  bring  each  stage  of  the  test 
into  as  nearly  like  conditions  of  attention  and  fatigue  as 
possible ;  at  least,  when  accuracy  is  an  object.  A  neces- 
sary condition  of  this  method  of  compensating  constant 
error  is  that  the  sets  of  tests  combined  shall  be  of  equal 
extent,  and  as  much  alike  as  possible  in  all  respects  except 
the  variation  especially  intended. 

In  tests  with  the  Method  of  Minimal  Change  the  subject 
is  supposed  to  know  the  direction  in  which  the  variable 
stimulus  is  altered,  but  not  the  precise  amount.  He  is 
therefore  liable  to  the  effects  of  expectant  attention.  Be- 
sides the  danger  of  reaching  the  expected  result  too  soon, 
which  is  guarded  against  by  combining  the  determinations 
of  just  observable  and  just  unobservable  differences,  there 
is  also  a  certain  danger  that  the  responses  may  after  a 
time  become  mechanical,  and  the  subject  report  the  limen 


238]  WEBER'S  PSYCHOPHYSIC  METHODS.  349 

after  a  certain  number  of  applications  of  the  variable  stim- 
ulus, irrespective  of  its  amount.  This  may  be  met  by 
occasional  changes  from  the  regular  procedure  —  changing 
the  size  of  the  alterations  by  which  the  limen  is  ap- 
proached, repeating  the  standard  or  one  of  the  inter- 
mediate weights,  etc.  Economy  of  time  and  patience 
dictates  that  the  alterations  of  the  variable  stimulus 
should  be  large  while  remote  from  the  limen  and  small 
when  near  it,  and  that  the  number  should  not  be  so  great 
as  to  weary  the  subject  before  reaching  it.  The  number 
will  of  course  vary  somewhat  with  circumstances ;  Kiilpe 
speaks  of  five  as  an  average  number.  Other  precautions 
will  suggest  themselves  to  the  careful  experimenter  as  need 
arises. 

The  chief  criticism  passed  upon  the  method  is  that  its 
criterion  of  the  just  noticeable  difference  is  wholly  subjec- 
tive, and  will  vary  with  the  subject's  willingness  to  risk 
errors.  This  is  undoubtedly  true,  but  does  not  rob  the 
method  of  all  usefulness. 

238.  The  Method  of  Eight  and  Wrong  Cases.  This 
method  may  be  tried  conveniently  with  visual  extents  or 
lifted  weights,  and  will  be  illustrated  here  by  both.  In 
applying  it.  a  certain  difference  of  stimuli,  known  to  be 
generally,  but  not  always,  recognizable,  is  chosen,  and  the 
variable  stimulus  set  once  for  all  greater  or  less  than  the 
standard  by  that  amount.  The  two  stimuli  are  then  pre- 
sented to  the  subject  a  large  number  of  times  for  judg- 
ment. Sometimes  the  greater  stimulus  will  seem  greater, 
sometimes  it  will  be  indistinguishable,  sometimes  it  will 
seem  less.  On  the  assumption  that  these  variations  are 
of  the  same  sort  as  those  contemplated  by  the  mathemat- 
ical theory  of  errors,  methods  of  calculation  have  been 
worked  out  which  give  values  somewhat  analogous  to  those 
reached  by  the  Method  of  Minimal  Change,  and  by  which 


350       LABORATORY  COURSE  IN  PSYCHOLOGY.      [238 
> 

also  it  is  possible  to  calculate  from  the  ratio  of  right 
judgments  and  the  difference  of  the  stimuli  in  a  given  case 
the  difference  required  to  give  any  other  proportion  of 
right  judgments  under  similar  circumstances.  These  so 
far  as  they  are  necessary  for  the  treatment  of  ordinary  re- 
sults will  appear  in  the  following  examples. 

In  the  first  illustration  the  method  will  be  followed  in 
its  classical  form ;  in  the  second,  in  a  simpler  form  recom- 
mended by  Jastrow,  and  Fullerton  and  Cattell. 

For  the  first  example,  let  us  suppose  that  in  an  experi- 
ment with  visual  extents  a  standard  stimulus  of  1.5  inches, 
and  a  variable  stimulus,  different  by  .03  of  an  inch,  were 
selected ;  that  two  hundred  judgments  were  made,  one  hun- 
dred with  the  variable  stimulus  greater,  and  an  equal  num- 
ber with  it  less ;  that  the  standard  lay  at  the  right  side  in 
one-half  of  each  set,  and  at  the  left  in  the  other  half,  and 
that  the  following  records  of  right,  wrong,  and  equal  cases 
were  made :  — 

Standard  stimulus,  1.50  inches,  variable  stimulus,  1.53  inches. 

I.  STANDARD  RIGHT.  II.  STANDARD  LEFT. 

Right,     31.  Right,     31. 

Wrong,    5,  Wrong,    2. 

Equal,    14.  Equal,    17. 

Standard  stimulus,  1.50  inches,  variable  stimulus,  l.£7  inches. 

III.  STANDARD  BIGHT.  IV.  STANDARD  LEFT. 

Right,     25.  Right,     18. 

Wrong,    1.  Wrong,    6. 

Equal,    24.  Equal,    26. 

The  values  which  this  method  gives  are  the  measure  of 
precision,  commonly  represented  by  h  in  the  formulae, 
which  is  directly  proportional  to  the  keenness  of  discrimi- 
native sensibility,  and  a  value  for  the  average  limen ;  or,  as 
it  may  very  well  be  termed  when  reached  by  this  method, 


238] 


THE  PSTCHOPHYSIC  METHODS. 


351 


the  "probable  limen"  ($),  though  the  trustworthiness  of 
this  value  is  questioned.1  These  may  be  reached  in  any 
special  case  from  the  following  formulae  and  table,  D  being 
the  difference  of  the  stimuli  employed,  tl  the  value  of  t 
corresponding  to  the  percentage  of  right  cases,  and  tz  that 
corresponding  to  the  percentage  of  right  and  equal  cases 

combined. 

/    i  /  •/•       -f 

h  = 


FECHNER'S  FUNDAMENTAL  TABLE.2 


r 

rf 

t=hD 

r 

W 

t  =  hD 

r 

li 

t  =  hD 

r 
n 

t=hD 

r 

^n 

t  =  hD 

.50 

.0000 

.60 

.1791 

.70 

.3708 

.80 

.5951 

.90 

.9062 

.51 

.0177 

.61 

.1975 

.71 

.3913 

.81 

.6208 

.91 

.9481 

.52 

.0355 

.62 

.2160 

.72 

.4121 

.82 

.6473 

.92 

.9936 

.53 

.0532 

.63 

.2347 

.73 

.4333 

.83 

.6747 

.93 

1.0436 

.54 

.0710 

.64 

.2535 

.74 

.4549 

.84 

.7032 

.94 

1.0994 

.55 

.0890 

.65 

.2725 

.75 

.4769 

.85 

.7329 

.95 

1.1631 

.56 

.1068 

.66 

.2917 

.76 

.4994 

.86 

.7639 

.96 

1.2379 

.57 

.1247 

.67 

.3111 

.77 

.5224 

.87 

.7965 

.97 

1.3297 

.58 

.1428 

.68 

.3307 

.78 

.5460 

.88 

.8308 

.98 

1.4522 

.59 

.1609 

.69 

.3506 

.79 

.5702 

.89 

.8673 

.99 

1.6450 

1.00 

GO 

In  the  example,  when  the  standard  was  at  the  right  and 
the  stimuli  were  150  and  153  (taking  0.01  inch  as  the  unit 
for  convenience  in  calculation),  the  number  of  right  cases 
was  31,  and  of  equal  14,  or  in  per  cent.  62  and  28.  The 

1  From  h  also  may  be  reckoned,  the  "  probable  error  "  (see  below,  p.  359,  note) 
to  which,  it  stands  in  a  fixed  relation  ;  namely, 


Klilpe  (p.  69)  defines  S  as  "  that  stimulus  difference  which  is  just  as  often 
cognized  (correctly  judged)  as  not  cognized  (incorrectly  judged)." 

2  Condensed  from  Wundt's  "  Physiologische  Psychologic,"  4te  Aufl.,  T.,  350. 


352       LABORATORY  COURSE  IN  PSYCHOLOGY.      [238 

value  of  t  for  62%  is  found  in  the  table  as  0.2160,  and 
that  for  90%  (i.e.,  62  +  28)  as  0.9062.  Substituting  these 
values  for  t±  and  t2  in  the  formula  for  A,  and  dividing  by 
2  D  (=  6),  we  have  h  =  0.19.  Substituting  in  the  formula 
for  S  we  have  1.85.  Applying  the  same  formulae  in  the 
case  when  the  standard  was  at  the  left  gives  the  values 
h  =  0.24  ;  S  =  2.11.  In  the  third  case  (stimuli  150  and 
147,  standard  right),  the  only  difference  is  that  the  per- 
centage of  right  cases  is  only  50,  and  consequently  ^  =  0. 
The  values  of  h  and  S  are  :  h  =  0.27  ;  S  =  3.  In  the  fourth 
case  (standard  left),  the  number  of  right  cases  is  only  18. 
This  shows  the  presence  of  a  constant  error  of  greater 
amount  than  the  difference  used.  The  effective  value  of 
the  standard  stimulus  is  less  than  that  of  the  variable. 
The  values  of  h  and  S  may  be  calculated,  however,  by 
taking  the  percentage  by  which  the  right  cases  fall  short 
of  100,  and  using  hD  as  a  minus  quantity.  In  this  case 
18  right  judgments  are  36%  ;  100  —  36  =  64,  the  value  of 
t  for  which  is  0.2535,  or  here  tl  =  —  0.2535.  The  value 
of  tz,  found  in  the  usual  way,  is  0.8308,  which  substituted 
in  the  formulae  with  t±  gives  :  h  =  .10  ;  S  —  5.63. 

It  is  clear  that  not  only  in  this  last  case,  but  in  the 
others  also,  there  is  a  constant  error,  and  consequently  that 
the  values  of  h  and  S  are  not  what  they  should  be.  It  was 
with  the  elimination  of  these  in  view  that  the  conditions 
were  varied,  both  as  to  the  position  of  the  standard  and 
the  size  of  the  variable  stimulus,  in  such  a  way  that  both 
influences  should  be  opposed  to  themselves  in  an  equal  num- 
ber of  cases.  The  average  result  is  therefore  nearer  right 
than  any  of  the  individual  ones.  Averaging  gives  h  =  0.20, 
S  =  3.15.  Expressed  in  a  formula,  this  process  stands  :  — 

,        Tt  +  T,  +  T,  +  T4 

k=          -TIT     ~> 


238]  THE  PSYCHOPHYS1C  METHODS.  358 

T  in  this  case  representing  the  combined    l  "T   2  of  pre- 
vious formulae. 

By  similar  formula  the  values  of  the  two  constant  errors 
allowed  for  in  the  arrangement  of  the  tests  may  be  found. 
If  we  let  Cf  represent  the  constant  tendency  which  increases 
the  apparent  difference  when  the  standard  lies  at  the  right, 
and  C"  the  tendency  which  increases  the  apparent  differ- 
ence when  the  standard  is  compared  with  a  larger  variable 
(150  with  153),  it  is  clear  that  in  Group  I.  both  C'  and 
C"  will  be  positive;  in  Group  II.,  C'  will  be  negative  and 
C"  positive;  in  Group  III.,  C'  positive  and  C"  negative; 
and  in  Group  IV.,  both  C'  and  C"  will  be  negative.  From 
these  relations  are  derived  the  following :  — 

c,  =  T,  -  T2  +  T8  -  T,  ^  D 
~  T!  +  T2  +  Ts  +  Tt  " 

C"=       ~^~        —        —          D 
~  T,  +  Tz  +  Ts  +  T4  ' 

Making  the  calculation  in  this  case  gives   C'  =  0.46  and 
C"  =  0.22.1 

The  Method  of  Right  and  Wrong  Cases  in  its  Simpler 
Form.  Much  complexity  is  introduced  into  the  classical 
form  of  the  method  by  the  cases  which  the  subject  pro- 
nounces "  equal."  In  the  simpler  form  advocated  by  Jas- 
trow,  and  Fullerton  and  Cattell,  all  these  are  avoided  by 
requiring  the  subject  to  indicate  one  or  the  other  of  the 
stimuli  each  time  as  greater,  and  in  case  of  complete  un- 
certainty to  guess.  The  resulting  record  contains  right 
and  wrong  judgments  only.  From  the  percentage  of  right 

1  The  data  of  this  example  are  the  records  of  an  actual  experiment,  made, 
however,  under  not  altogether  constant  conditions,  and  not  such  as  would  favor 
great  accuracy.  They  serve,  however,  well  enough  for  illustrating  the  method 
of  calculation.  Under  favorable  circumstances  h  might  be  expected  to  be 
larger  and  S  smaller. 


354      LABORATORY  COURSE  IN  PSYCHOLOGY.      [238 

cases,  taken  in  connection  with  the  difference  of  the  stimuli 
employed,  the  amount  of  difference  required  to  give  75% 
of  right  judgments  is  calculated.  This  is  regarded  as  the 
value  of  the  average  or  probable  limen,  and  its  ratio  to  the 
standard  used  like  that  of  the  limen  in  the  Method  of  Min- 
imal Change  as  the  measure  of  the  keenness  of  sensibility. 
The  selection  of  75%  is  of  course  arbitrary,  but  it  has 
certain  recommendations.  It  lies  midway  between  the  pro- 
portion of  cases  right  by  pure  chance  (50%)  and  the  pro- 
portion right  in  case  of  full  certainty,  and  corresponds  also 
to  the  value  of  the  "  probable  error "  in  the  probability 
curve.  To  facilitate  this  calculation,  Fullerton  and  Cattell 
give  the  table  below.1 


TABLE   FOR  DETERMINING    THE   PROBABLE    ERROR  FROM 

THE   PERCENTAGE    OF   RIGHT   CASES   AND  AMOUNT 

OF  DIFFERENCE. 


D 

• 

D 

D 

r 

D 

r 

D 

p.e. 

p.e. 

p.  e. 

p.e. 

p.e. 

50 

.00 

60 

.38 

70 

.78 

80 

1.25 

90 

1.90 

51 

.04 

61 

.41 

71 

.82 

81 

1.30 

91 

1.99 

52 

.07 

62 

.45 

72 

.86 

82 

1.36 

92 

2.08 

53 

.11 

63 

.49 

73 

.91 

83 

1.41 

93 

2.19 

54 

.15 

64 

.53 

74 

.95 

84 

1.47 

94 

2.31 

55 

.19 

65 

.57 

75 

1.00 

85 

1.54 

95 

2.44 

56 

.22 

66 

.61 

76 

1.05 

86 

1.60 

96 

2.60 

57 

.26 

67 

.65 

77 

1.10 

87 

1.67 

97 

2.79 

58 

.30 

68 

.69 

78 

1.14 

88 

1.74 

98 

3.05 

59 

.34 

69 

.74 

79 

1.20 

89 

1.82 

99 

3.45 

The  use  of  the  table  will  appear  in  the  following  exam- 
ple taken  from  their  records  (p.  121).  The  experiments 

1  It  is  obvious  from  the  relation  between  the  probable  error  and  the  quantity 
ft  already  mentioned  I  p.e.  =  -^ — \  that  this  table  can  be  made  out  by  dividing 
the  corresponding  values  of  Fechner's  table  by  0.4769. 


238]  THE  PSYCHOPHYXIC  METHODS.  .355 

were  on  the  discrimination  of  lifted  weights.  In  a  set  of 
50  trials,  when  the  weights  used  were  100  and  104  grams, 
and  the  heavier  weight  was  lifted  first,  the  subject  E,  made 
36  right  judgments  and  14  wrong  ;  in  an  equal  number,  with 
the  same  weights  lifted  in  the  reverse  order,  the  right  judg- 
ments were  28  and  the  wrong  22,  or  in  percentage  72  % 
right  with  the  standard  following,  and  56%  right  with  the 

standard  leading.     Referring  to  the  table,  we  find  for  — • 

p.e. 

in  the  first  instance  0.86,  which  means  that  in  order  to  give 
the  proportion  of  75%  of  right  judgments  the  difference 
(4  grams)  would  (probably)  have  to  be  increased  in  the  ratio 
of  0.86  : 1 ;  that  is,  to  4.65  grams  —  this,  however,  only  in 
case  the  heavier  weight  comes  first.  When  it  came  last  the 
percentage  of  right  judgments  was  56,  which  corresponds 
in  the  table  to  0.22 ;  and  the  difference  would  have  to  be 
increased  in  the  ratio  of  0.22  : 1,  that  is,  to  18.18  grams. 
There  is  then  a  strong  tendency  toward  underestimation  of 
the  second  weight.1  This  can  be  eliminated,  however,  in  a 
manner  similar  to  that  indicated  in  the  previous  example. 

Representing  the  constant  error  by  C,  the  in  the  first 

..     .     D  +  C  D-C 

case  would  really  be ,  and  in  the  second  case—      —  • 

p.e.  2D  p.e. 

Adding  these  fractions    eliminates    (7,   giving    -  — ,    from 

p.e. 

which  it  is  easy  to  get  the  D  required  for  75%  right 
judgments.  0.86  +  0.22  =  1.08  and  1.08  -f-  2  =  0.54, 
which  gives  7.4  grams  as  the  amount  required  when  con- 
stant errors  are  excluded.  This  is  the  average  limen,  and 
the  sensibility  of  the  subject  may  be  expressed  by  its 
reciprocal,  ^ ,  or  by  its  ratio  to  the  standard,  7.4  : 100. 
If  the  value  of  the  constant  error  (in  this  case  the  time 

1  "With  most  of  Fullerton  and  Cattell's  subjects  the  tendency  was  in  the  other 
direction,  and  that  would  appear  to  be  the  common  tendency. 


856       LABORATORY  COURSE  IN  PSYCHOLOGY.     [238 

error)  is  desired,  it  can  be  calculated  after  the  same  anal- 

D  ±  C      D-  C  =  2  C 
p.e.  p.e.          p.e. 

2  C  C 

In  this  instance  —  =  0.86  —  0.22  =  0.64.     —  =  0.32. 
p.e.  D  p.e.          D 

It  has  "just  been  found  that  —  =  0.54,  whence  p.e.  =  ^-^— 

032  p'e-  °'54 

and   C  =  p-Vr  •  D  =  2.37.        The    elimination    of   several 
0.54 

simultaneous  constant  errors  would  of  course  be  the  same 
in  principle,  but  the  experiment  would  have  to  be  planned 
with  that  in  view  from  the  start. 

It  remains  to  speak  of  a  few  points  with  regard  to  the 
actual  use  of  the  method  in  both  its  forms.  The  first 
requisite  is  a  large  number  of  tests ;  the  formulae  are  not 
very  reliable  when  the  numbers  are  small.  If  long  series 
are  fatiguing,  several  shorter  series  taken  under  like  condi- 
tions may  be  combined.  Such  a  difference  between  the 
stimuli  must  be  chosen  if  possible  as  will  give  neither  too 
great  nor  too  small  a  number  of  right  judgments  —  about 
eighty-five  in  a  hundred  is  recommended  by  Fullerton  and 
Cattell.  It  is  therefore  profitable  to  make  preliminary  de- 
terminations by  one  of  the  other  methods  before  undertak- 
ing systematic  experiments  by  this. 

The  method  has  been  used  when  the  subject  had  full 
knowledge  of  the  actual  relation  of  the  stimuli  (when  he 
was  himself  the  operator),  but  this  usage  is  less  to  be 
recommended  than  that  in  which  the  subject  is  in  complete 
ignorance.  This  is  absolutely  essential,  indeed,  in  the  sim- 
pler form  of  the  method. 

The  subject's  task  is  simply  to  judge  of  the  stimuli  pre- 
sented to  him,  the  operator's  to  present  the  stimuli  as  uni- 
formly as  possible.  In  the  simpler  form  of  the  method 
the  subject  will  be  assisted  in  his  selection  of  one  or  the 


239]  THE  PSYVHOPHY8IC  METHODS.  357 

other  as  greater  by  a  clear  understanding  that  he  is  never 
to  be  presented  with  equal  stimuli.1  He  must  also  feel 
sure  that  one  arrangement  is  as  likely  to  be  given  as  the 
other,  lighter-heavier  as  likely  to  occur  with  weights,  for 
example,  as  heavier-lighter.  In  short  series  the  subject  is 
apt  to  be  influenced  by  his  recollection  of  how  often  his 
answer  has  fallen  one  way  or  the  other;  but  this  can  be 
met  by  longer  series,  or  by  short  series  so  arranged  that 
the  number  of  times  each  arrangement  occurs  shall  not  be 
the  same  in  each  series,  but  that  the  final  combined  record 
shall  show  an  equal  number  of  tests  made  with  each  ar- 
rangement.2 

It  is  not  always  convenient  to  present  a  standard  stimu- 
lus with  two  variable  stimuli,  one  larger  and  one  smaller, 
as  was  done  in  the  example  with  visual  extents,  and  some- 
times it  entails  great  waste  of  time.  It  is  advisable,  there- 
fore, in  ordinary  experiments  to  carry  out  the  testing  with 
two  stimuli  only,  letting  the  greater  serve  as  the  standard 
when  experimenting  for  the  determination  of  the  probable 
limen  below,  and  the  less  when  experimenting  for  that 
above,  as  in  the  second  example.  For  other  suggestions 
and  precautions,  see  Jastrow,  B. 

239.  The  Method  of  Average  Error.  This  method  is 
applicable  to  any  stimuli  that  can  be  put,  directly  or  in- 
directly, under  the  control  of  the  subject ;  it  is  most  con- 
venient when  the  control  is  direct  and  simple.  These 
conditions  are  fully  met  in  the  case  of  the  comparison  of 
lengths  by  vision;  for  example,  in  experiments  with  the 


1  If  for  any  reason  a  few  doubtful  answers  cannot  be  avoided,  they  may  be 
divided  equally  between  the  right  and  wrong  judgments  without  serious  error, 
but  it  is  better  not  to  allow  them  at  all. 

1  A  similar  difficulty  is  met  in  the  case  of  large  constant  errors  which  throw 
a  marked  proportion  of  the  judgments  in  one  direction  or  the  other.  This  is 
more  difficult  to  deal  with,  but  might  be  helped  by  a  special  combination  of 
series  with  stimuli  of  different  degrees  of  difference. 


358       LABORATORY  COURSE  IN  PSYCHOLOGY.     [239 


Galton  bar,  or  with  narrow  strips  of  millimeter  paper,  cut 
through  with  a  knife  point.1  The  application  of  the 
method  is  so  simple  that  it  may  be  easily  understood  from 
a  single  example.  One  slide  of  an  instrument  somewhat 
like  the  Galton  bar  was  set  at  a  standard  distance  from 
the  centre  line  of  1.50  inches.  The  instrument  was  then 
handed  the  subject,  with  the  request  that  he  set  the  other 
slide  at  exactly  the  same  distance  from  the  line.  When  he 
had  done  so  the  instrument  was  examined,  his  setting  re- 
corded, the  slide  displaced,  and  the  whole  process  again 
repeated.  Forty  trials  were  made,  in  half  of  which  the 
standard  distance  lay  at  the  right,  and  in  half  at  the  left. 
In  half  the  trials  in  each  position,  also,  the  movable  slide 
was  too  far  from  the  centre  line  when  the  instrument  was 
given  to  the  subject,  and  in  half  it  was  too  near.  The  fol- 
lowing settings  were  regarded  by  the  subject  as  equal  to 
the  standard  of  1.50  inches.  The  unit  in  the  table  and  in 
subsequent  calculation  is  taken  for  convenience  as  0.01 
inch. 


STANDARD  LEFT. 

STANDARD  RIGHT. 

FROM  TOO  SMALL. 

FROM  TOO  LARGE. 

FROM  TOO  SMALL. 

FROM  TOO  LARGE. 

147 

145 

144 

147 

145 

145 

149 

148 

145 

150 

145 

146 

147 

148 

148 

151 

149 

149 

150 

146 

145 

151 

148 

151 

149 

149 

146 

150 

142 

150 

149 

156 

147 

151 

147 

149 

151 

144 

146 

147 

1  These  strips  may  be  200  mm.  long  by  5  mm.  wide.    When  in  use  they  are 
presented  to  the  subject  with  the  blank  side  of  the  strip  uppermost,  and  he  is 


239]  THE  PSYCHOPHYSIC  METHODS.  359 

The  average  of  these  forty  trials  is  147.8,  showing  a  con- 
stant error  of  2.2 ;  that  is,  the  extent  made  by  the  subject 
fell  2.2  short  on  the  average,  or,  in  other  words,  he  over- 
estimated his  setting  each  time  by  that  amount  on  the 
average.  The  mean  variation  of  the  single  settings  from 
the  average,  found  as  explained  above,  is  2.11,  the  average 
limen  (S)  as  given  by  this  method.  Its  reciprocal  -—  is 
the  index  of  the  keenness  of  sensibility.  Expressed  as  a 
ratio,  it  is  2.11  :  150,  or  about  1  :  71 ;  in  per  cent,  of  the 
standard  1.41.1 

If  we  take  the  groups  separately  we  can  find  the  effect 
of  the  special  conditions  to  which  each  was  submitted.  The 
averages  are  as  follows  :  — 

STANDARD    LEFT.  STANDARD    RIGHT. 

From  too  small      146.7  From  too  small      147.2 

From  too  large       148.2  From  too  large       149.1 

Combination  of  these  shows  that  the  average  setting 
when  the  standard  was  at  the  left  was  147.45,  and  when 

asked  to  indicate  by  a  sharp  cut  with  the  point  of  a  penknife  the  exact  middle 
of  the  strip.  The  strip  is  then  turned  over,  and  the  displacement  of  the  cut 
from  the  exact  centre  is  easily  read  in  millimeters  and  (by  estimate)  to  tenths 
of  a  millimeter. 

1  The  mean  variation  is  not  the  only  quantity  which  can  be  taken  as  the 
limen  by  this  method  ;  the  average  error,  and  the  probable  error  of  a  single 
observation  may  both  serve  that  purpose.  The  average  error  is  not  to  be  con- 
fused with  the  average  (or  mean)  variation,  the  two  standing  to  one  another 
in  the  following  proportion  :  — 

M.  V.  :  A.  E.  :  :  1 : 1.2533. 

The  probable  error  of  a  single  observation  is  given  by  the  formula 

or  approximately  by  the  formula  :  — 

_  0.84532V 


in  which  2,  v,  and  n  have  the  same  significance  as  before. 
Its  relation  to  the  mean  variation  is  roughly  :  — 


360      LABORATORY  COURSE  IN  PSYCHOLOGY.     [239 

at  the  right,  148.15.  Half  of  the  difference  between  these 
two  is  0.35,  and  is  the  amount  fairly  to  be  credited  to  the 
space  error,  —  the  amount  by  which,  on  the  average,  the 
subject  overestimated  the  variable  extent  when  comparing 
it  with  a  standard  of  150  lying  at  the  left. 

Combining  the  series  in  which  the  direction  of  the  move- 
ment of  the  slide  was  different,  we  get  an  average  of  146.95 
when  the  slide  was  too  near  at  the  start,  and  148.65  when 
it  was  too  far  away.  The  subject  evidently  tended  to 
stop  too  soon  in  the  moving  in  both  cases,  and  by  an  amount 
equal  to  half  the  difference  of  these  averages,  namely,  by 
0.85,  —  a  space  error  of  another  kind,  and  in  this  instance 
a  more  important  one. 

The  constant  error  of  the  general  average  (2.2),  which 
remains  in  spite  of  the  combination  of  all  four  groups,  in- 
dicates the  action  of  some  other  unneutralized  cause,  and, 
if  all  other  influences  have  been  certainly  avoided,  to  some 
peculiarity  of  the  subject's  method  of  estimate.1  To  deter- 
mine whether  the  cause  of  the  constant  error  lies  in  some 
truly  constant  tendency,  or  in  mere  accident  (as  it  may 
when  the  number  of  trials  is  small),  the  probable  error  of 
the  average  (147.8)  must  be  calculated.2  If  the  constant 

1  This  example,  like  those  before,  is  the  record  of  an  actual  experiment. 
Unfortunately  the  data  are  not  sufficient  to  exclude  the  possibility  that  at  least 
a  part  of  this  2.2  may  be  of  instrumental  origin.    The  ratio  of  the  general  limen 
to  the  standard  (1 : 71)  is  larger  also  than  is  usually  expected  in  experiments 
of  this  kind. 

2  The  probable  error  of  the  average  is  calculated  according  to  the  following 
formula  :  —  . 

p.e.  =  0.6745  ^J     S^ 

For  such  use  as  is  here  contemplated,  the  approximate  formula  :  — 


-  - 

will  answer  every  purpose. 

Merriman's  Method  of  Least  Squares,  from  which  these  and  several  of  the 
preceding  formula}  are  taken,  also  contains  tables  by  which  the  calculation  of 
the  probable  error  is  much  facilitated. 


239]  THE  PSYCHOPHYSIC  METHODS.  361 

error  is  distinctly  larger  than  the  probable  error  of  the  aver- 
age, it  may  be  assumed  to  represent  a  true  tendency,  and 
not  an  accident.  In  this  instance  the  probable  error  is  0.28. 

The  Method  of  Average  Error  and  the  Method  of  Eight 
and  Wrong  Cases  are  both  founded  on  the  assumption  that 
the  principle  of  probability  applies  to  the  small  uncertain- 
ties or  variations  of  perception.  The  results  obtained  by 
them  ought,  therefore,  to  be  comparable,  and,  provided  the 
conditions  are  the  same,  not  greatly  different  in  amount. 
A  difference  in  result  would  therefore  point  to  a  difference 
in  conditions,  and,  external  conditions  being  the  same,  to 
those  of  a  subjective  character.  The  two  experiments  with 
visual  extents,  used  as  illustrations  above,  were  made  wi 
the  same  subject  and  same  apparatus,  and  may  serve  for 
illustration  of  this  point  also.  There  are  two  quantities 
among  the  results  that  may  be  used  for  comparison  :  the 
difference  required  for  75%  of  right  judgments,  which 
should  be  the  same  as  the  probable  error,  and  the  meas- 
ure of  precision,  the  quantity  h  of  the  formulae.1  By  the 
Method  of  Eight  and  Wrong  Cases  we  had  h  —  0.20,  giv- 
ing^?, e.  2.38.  By  the  Method  of  Average  Error  we  have 
h  =  0.26,  and  p.  e.  1.81,  showing  a  finer  discrimination  in 
the  latter  case,  which  in  this  instance  may  well  correspond 
to  a  higher  grade  of  attention. 

Of  special  precautions  in  the  use  of  this  method  little 
need  be  said.  The  subject  should,  of  course,  remain  in 
ignorance  of  the  amount  and  nature  of  his  errors:  The 
groups  of  tests  should  be  so  arranged  as  to  compensate  con- 
stant errors.  The  variable  stimulus  should  be  made  to 
approach  equality  an  equal  number  of  times  from  both 
above  and  below,  especially  when  the  control  of  it  is  in- 
direct and  the  subject  must  order  changes  made  instead 


1  The  constant  errors,  so  far  as  they  arise  from  the  same  causes,  might  also 
!>e  eomj.ared.  The  probable  error  above  referred  to  is,  of  course,  the  probable 
error  of  a  single  observation. 


362      LABORATORY  COURSE  IN  PSYCHOLOGY.      [239 

of  making  them  himself.  The  number  of  tests  will  vary 
with  the  purpose  of  the  experiments.  The  formulae  for 
the  probable  error  apply  more  exactly  as  the  number  of 
observations  increases,  but  the  number  necessary  in  this 
method  is  much  less  than  in  the  Method  of  Right  and 
Wrong  Cases.  The  method  has  very  great  practical  advan- 
tages in  the  simplicity  of  its  procedure  and  in  requiring 
discrimination  not  by  itself  alone,  but  in  its  natural  rela- 
tion to  action. 


BIBLIOGRAPHY. 

AUBERT  :  Grundziige  der  physiologischen  Optik,  Leipzig,  1876. 
BOWDITCH  :  "  Vision,"  American  Text-Book  of  Physiology,  Philadelphia, 

1896,  especially  pp.  794-796. 
DELBOEUF:  Etude  psychophysique,  Memoires  couronnes,  etc.,  de  1'Aca- 

demie  royale  de  Belgique,  1873.     Contained  also  in  his  Elements  de 

Psychophysique,  Paris,  1883. 
FULLER-TON  AND  CATTELL:  On  the  Perception  of  Small  Differences. 

Publications  of  the  University  of  Pennsylvania,  Philosophical  Series, 

No.  2,  May,  1892. 
HELMHOLTZ:  Handbuch  der  physiologischen  Optik,  2te  Aufl.,  Hamburg 

und  Leipzig,  1886-96.     French  translation  of  first  edition,  Optique 

physiologique,  Paris,  1867. 
HERING:  Ueber  Irradiation,  Hermann's  Handbuch  der  Physiologie,!!!., 

ii.,  440-448. 
JASTROW:  A.    The  Psycho-physic  Law  and  Star  Magnitudes,  American 

Journal  of  Psychology,  I.,  1887-88,  112-127. 

B.  A  Critique  of  Psycho-physic  Methods,  ibid.,  271-309. 

C.  Studies  from  the  Laboratory  of  Experimental  Psychology  of  the 
University  of  Wisconsin,  ibid.,  III.,  1890-91,  44-49,  54-58;  IV.,  1891- 
92,  213-219. 

KIRSCHMANN:   Color-saturation  and.  its  Quantitative  Relations,   ibid., 

VII.,  1895-96,  386^04. 
KiJLPE :  Outlines  of  Psychology,  translation  by  Titchener,  London  and 

New  York,  1895. 
LEUBA  :  A  New  Instrument  for  Weber's  Law,  with  Indications  of  a  Law 

of  Sense  Memory,  American  Journal  of  Psychology,  V.,  1892-93, 

370-384. 
WUNDT:  Grundziige  der  physiologischen  Psychologic,  4te  Aufl.,  Leipzig, 

1893. 


SUGGESTIONS  ON  APPARATUS,  363 


CHAPTER    IX. 
Suggestions  on  Apparatus. 

THE  aim  in  the  foregoing  chapters  has  been,  almost 
without  exception,  to  work  with  the  simplest  and  least 
expensive  apparatus  that  gave  promise  of  bringing  out 
the  essential  phenomena  of  the  experiment.  That  the 
apparatus  used  has  always  been  the  best,  even  from  this 
standpoint,  is  far  from  the  writer's  conception.  Some 
apparatus  must  be  used,  however,  and  the  following  sug- 
gestions are  offered  for  what  they  may  be  worth. 

It  is  assumed  that  most  of  the  simpler  pieces,  especially 
those  of  paper  and  cardboard,  will  be  prepared  in  the  labo- 
ratory, and  that  the  more  difficult  and  complicated  pieces 
will  be  bought  outright,  or  made  by  a  skilled  mechanic. 
Where  specifications  are  given,  they  are  for  the  most  part 
taken  from  the  pieces  actually  used  in  the  Clark  labora- 
tory, except  where  decided  improvements  have  suggested 
themselves.  This  is  done  to  make  the  descriptions  as  defi- 
nite and  helpful  as  possible  ;  but  improvements  in  con- 
struction may  frequently  suggest  themselves  to  the  reader, 
and  ought  not  to  be  disregarded. 

Apparatus  for  Chapter  I.,  on  the  Dermal  Senses, 
Experiments  1-32. 

In  the  way  of  general  apparatus  and  supplies  will  be 
needed  a  millimeter  scale,  a  cane  or  light  rod,  a  bit  of 
metric  cross-section  paper,  vessels  and  means  for  heating 
water,  two  large  vessels  of  the  same  size  and  shape,  some 


364        LABORATOEY  COURSE  IN  PSYCHOLOGY. 

bits  of  cork,  a  piece  of  ice,  court-plaster,  a  little  ether,  a 
menthol  pencil,  and  three  thermometers  (preferably  centi- 
grade). 

Of  special  apparatus  will  be  needed  a  compass  sesthesi- 
ometer,  an  ether  spray,  a  tuning-fork  (see  apparatus  for 
Sensations  of  Hearing),  temperature-spot  seekers,  apparatus 
for  making  C02,  a  centigrade  thermometer,  reading  in 
tenths  of  a  degree  between  zero  and  55°,  weights  for 
minimal  pressure  and  for  discriminative  sensibility,  equal 
weights  of  unequal  size  for  pressure,  wooden  cylinders  of 
equal  weight,  and  an  algometer. 

The  Compass  JEstkesiometer.  —  An  instrument  of  this 
sort  has  long  been  used  by  physicians  and  others,  and  a 
good  many  forms  are  in  the  market.  An  ordinary  pair  of 
carpenter's  compasses,  to  be  had  at  a  low  price  at  any 
hardware  store,  will  answer  fairly  well,  the  separation 
of  the  points  being  measured  with  the  millimeter  scale 
when  required.  Drawing  dividers  can  also  be  used,  but 
are  rather  sharp,  and  will  serve  better  if  tipped  with 
pointed  bits  of  cork.  A  more  accurate  and  convenient 
pair  may  readily  be  made  by  a  slight  alteration  of  some 
of  the  callipers  or  calliper  squares,  to  be  found  in  the 
hardware  stores,  or  in  the  lists  of  the  dealers  in  physical 
instruments.1 

For  the  Ether  Spray  any  cheap  atomizer  will  answer. 

Simple  Temperature-point  Seekers  are  easily  made  by 
turning  down  round  brass  rods  (six  inches  long  and  a  quar- 
ter of  an  inch  in  diameter)  to  a  fine  smooth  point  (0.5  mm. 
in  diameter).  Large  wire  nails  may  be  used. 

On  Apparatus  for  making  C02  consult  any  text-book  on 
chemistry. 

1  For  more  elaborate  and  exact  apparatus,  see  Jastrow  [A  new  jesthesiometer], 
American  Journal  of  Psychology,  I.,  1887-88,  552,  and  Margaret  Floy  Washburn, 
Hid.,  VI.,  1898-95,  422. 


SUGGESTIONS  ON  APPARATUS.  365 

The  Thermometer  reading  to  0.1°  can  be  had  of  dealers 
in  chemical  supplies. 

Weights  for  Minimal  Pressure.  —  These  can  be  cut  from 
rectangular  prisms  of  cork  or  elder-pith  of  equal  end  area, 
and  provided  with  bristle  or  hair  handles,  and  verified  upon 
a  sensitive  balance.  The  prism  should  be  about  5  mm. 
square.  The  handle  is  made  by  setting  the  ends  of  a  piece 
of  bristle  or  hair  into  opposite  sides  of  the  bits  of  cork  or 
elder-pith,  thus  giving  the  whole  something  the  shape  of  a 
seal  ring,  of  which  the  cork  is  the  seal  and  the  bristle  the 
band.  A  series  ranging  from  2  to  20  mg.  would  be  conve- 
nient. Such  a  series,  using  paper  disks  instead  of  pith, 
has  been  prepared  by  Willyoung  of  Philadelphia,  after  the 
design  of  Dr.  Scripture. 

Equal  Weights  of  Unequal  Size  can  easily  be  made  from 
a  large  cork  and  a  small  one,  by  hollowing  out  the  small 
one  and  loading  it  with  lead.  Cf.  similar  apparatus  in  the 
section  on  Chap.  II. 

Weights  for  Testing  Discriminative  Sensibility  with  Pres- 
sures are  to  be  had  in  different  forms  from  the  dealers.  A 
set  can  be  made  in  the  laboratory  without  great  difficulty 
by  loading  shotgun  cartridges  with  shot,  as  suggested  by 
Galton.  Cf.  the  set  for  testing  discriminative  sensibility 
with  lifted  weights  in  the  section  on  Chap.  II. 

For  the  Wooden  Cylinders  used  in  Ex.  25  b,  bits  three- 
quarters  of  an  inch  long,  cut  from  the  end  of  a  broom- 
handle  and  made  smooth  on  the  ends,  and  of  equal  weight, 
would  probably  answer  as  well  as  anything. 

An  Algometer  is  not  necessary  for  the  experiments  de- 
scribed, but  has  now  become  a  well-recognized  instrument 
in  anthropometric  studies  of  sensibility,  and  may  be  had 
from  the  dealers.  The  common  one  in  this  country  is  that 
devised  by  Prof.  Cattell,  which  works  on  the  principle  of 
an  inverted  spring  balance. 


366       LABORATORY  COURSE  IN  PSYCHOLOGY. 


Apparatus  for  Chapter  II.,  on  the  Kineesthetic  and  Static 
Senses,  Experiments  33-51. 

Of  general  apparatus  for  this  section  there  is  little  addi- 
tional, except  a  meter  stick,  a  straw,  and  a  couple  of  small 
pieces  of  cotton  cloth. 

Of  special  apparatus  the  following  are  needed :  A  large 
weight,  equal  weights  of  unequal  size  (for  lifting),  weights 
for  testing  discriminative  sensibility  with  lifted  weights, 
a  joint-sense  board,  a  tilt-board,  and  a  rotation  table. 

Any  Large  Weight  that  can  be  conveniently  handled 
would  answer.  For  use  in  the  Clark  laboratory,  cylindri- 
cal weights  of  cast-iron,  three  inches  long  and  two  in 
diameter,  have  recently  been  made  and  are  satisfactory. 
They  weigh  between  two  and  three  pounds,  and  serve  very 
well  instead  of  the  two  kilogram  weight  mentioned  in 
Ex.  35.  If  bored  and  tapped  so  that  a  screw-eye  could  be 
inserted  in  the  middle  of  the  top,  they  would  be  convenient 
for  Ex.  43. 

The  Equal  Weights  of  Unequal  size  mentioned  above 
also  give  the  illusion  in  question  here.  If  for  any  reason 
it  is  desired  to  have  the  difference  of  size  in  one  dimension 
only,  a  set  can  easily  be  made  by  cutting  down  paper  car- 
tridges, and  then  loading  the  shorter  ones  till  they  equal 
the  longer  in  weight.  A  set  of  three  —  full  length  of  the 
cartridge,  two-thirds  length,  and  one-third  length  —  is  con- 
venient. In  making  these  it  is  well  to  put  corks  in  the 
longer  ones,  in  order  to  support  the  wads  with  which  all 
are  closed.  These  cartridge  weights  will  not  do  well  for 
Ex.  23.  Special  apparatus  for  this  illusion  has  been  de- 
vised by  Dr.  J.  A.  Gilbert,  and  is  to  be  had  under  the 
name  of  "  Suggestion  Blocks." 

Weights  for  Discriminative  Sensibility  in  the  case  of 
lifted  weights.  For  this  test  also  special  sets  have  been 


SUGGESTIONS   ON  APPARATUS.  367 

made  and  are  in  the  market.  Weights  are  easily  made, 
however,  from  cartridges,  as  before.  It  would  be  well  to 
make  the  set  large  enough  to  furnish  the  differences  re- 
quired for  Ex.  24,  as  well  as  the  finer  ones  for  Ex.  34. 
A  series  of  weights,  increasing  by  single  grams  from  80  to 
120  grams,  and  by  twos  from  60  to  80,  and  as  far  above 
120  as  the  cartridges  will  permit,  and  containing  two  100 
gram  weights,  ought  to  answer  every  purpose.  A  much 
smaller  set  will  do  if,  instead  of  keeping  the  standard  at 
100  grams  all  the  time,  a  standard  at  the  upper  end  of  the 
series  is  taken  when  experiments  are  to  be  made  below  the 
standard,  and  one  at  the  lower  end  when  experiments  are 
to  be  made  above  the  standard.  For  Ex.  34  the  weighted 
envelopes  of  Ex.  236  might  be  used,  but  they  would  not  do 
for  Ex.  24.  In  making  the  weights,  it  is  well  to  have  all 
seem  equally  filled  (which  may  be  done  by  using  cotton  as 
part  of  the  filling  in  the  lighter  cartridges),  and  to  so  dis- 
tribute the  cotton  or  other  light  substance  that  the  car- 
tridge will  not  have  all  the  weight  at  the  bottom.  The  first 
point  is  not  essential  if  the  subject's  eyes  are  closed  dur- 
ing the  experiment ;  and  the  second  is  unimportant,  except 
perhaps  with  the  lighter  weights,  if  they  are  lifted  in  a 
vertical  position. 

Joint-Sense  Board.  —  The  cut  accompanying  Ex.  39  will 
give  some  idea  of  the  construction  of  the  joint-sense  board. 
The  thin  board  on  which  the  fore-arm  rests  is  50  cm.  long 
by  8-10  wide,  and  is  hinged  at  one  end  to  the  base-board. 
At  the  other  end  a  cord  is  fastened,  which  runs  over  a 
pulley  upon  the  top  of  a  stout  post.  On  the  end  of  the 
cord  a  weight  is  hung  to  counterbalance  the  weight  of  the 
fore-arm.  A  scale  (e.g.,  a  piece  of  millimeter  paper)  on 
the  post  near  the  weight  enables,  the  experimenter  to  read 
off  the  distance  which  the  end  of  the  arm-board  is  raised 
or  lowered.  It  is  essential  that  the  hinge  and  pulley  work 
easily  and  without  jar. 


368      LABORATORY  COURSE  IN  PSYCHOLOGY. 

Tilt-Board.  —  For  part  of  the  experiments  with  the  tilt- 
board,  a  plank  seven  feet  long  and  eighteen  inches  wide, 
balanced  across  a  saw-horse,  will  answer.  For  others  a 
more  permanent  structure  is  necessary  ;  e.g.,  a  board  per- 
manently hinged  across  a  stout  horse  three  feet  and  a  half 
or  more  in  height,  and  with  wide-spreading  feet.  (For 
cut,  see  Ex.  46.)  If  the  top  piece  of  the  horse  is  not  too 
thick,  and  the  board  is  fastened  on  with  hinges  such  as 
are  used  for  doors  that  swing  both  ways,  it  will  be  possible 
to  get  all  required  positions  of  the  subject,  from  head  up- 
ward to  head  downward,  without  the  trouble  of  mounting 
the  board  on  an  axis.  At  one  end  of  the  long  board  a 
short  foot-board  should  be  fastened  securely  enough  to  bear 
the  weight  of  a  man  when  the  board  is  in  a  vertical  posi- 
tion. At  the  other  end  a  plumb-line  and  semi-circular  scale 
should  be  added,  so  that  the  inclination  of  the  board  can  be 
read  off  at  any  instant.  For  holding  the  subject  securely 
upon  the  board  when  its  inclination  is  considerable  and  the 
subject  is  head  downward,  it  will  be  necessary  to  have  a 
couple  of  straps  passing  over  the  subject's  shoulders  and 
fastening  to  stout  screw-eyes  screwed  into  the  board  itself 
or  into  the  foot-board,  and  perhaps  a  breast  strap  going 
about  both  the  subject  and  the  board. 

Rotation  Table.  —  A  table,  made  by  fastening  a  seven-foot 
plank  across  an  ordinary  turning-chair  or  screw-stool  with- 
out a  back,  could  probably  be  made  to  serve  for  some  of 
the  experiments.  The  apparatus  must  turn  without  appre- 
ciable noise  or  jar.  Many  of  the  experiments  could  be 
made  by  twisting  the  ropes  of  a  swing. 

The  accompanying  cut  shows  the  plan  of  a  permanent 
piece  devised  by  Aubert.1 


1  Physiologische  Studien  uber  die  Orientltrung ,  unter  Zugrundelegung  von 
Yves  Delage  Etudes,  etc.,  p.  50. 


SUGGESTIONS   ON  APPARATUS 


369 


The  plan  is  on  a  scale  of  about  one  twentieth.  The 
table  top  rests  on  a  central  spindle  of  steel,  the  lower  end 
of  which  fits  into  a  shallow  socket  in  the  lower  part  of  the 
iron  supporting-frame,  two  of  the  three  columns  of  which 


11 


are  shown  in  the  plan.  This  central  spindle  also  carries 
a  grooved  wheel.  The  table  is  turned  by  means  of  a  driv- 
ing-cord, which  passes  around  this  wheel,  and  a  small  drum, 
not  shown  in  the  plan,  which  is  turned  by  hand.  Aubert 
reports  the  instrument  a  very  satisfactory  one  with  regard 
both  to  ease  of  motion  and  freedom  from  jar. 

The  rotation  table  in  the  Clark  laboratory  differs  from 
this  in  being  constructed  entirely  of  wood  (the  supporting- 
frame  having  four  posts  instead  of  three),  and  in- the  omis- 
sion of  the  grooved  wheel.  The  direct  turning  by  hand, 
which  is  thus  required,  is  of  course  less  satisfactory  than 
turning  by  a  driving-cord,  but  answers  for  the  experiments 
described  in  the  text.  The  ends  of  the  table  top  are 
rounded,  the  curve  having  a  radius  equal  to  half  the  length 
of  the  table.  On  these  a  scale  of  degrees  might  well  be 
laid  off,  so  that  a  rough  judgment  of  the  angular  rate  would 
be  possible. 

Apparatus  for  Chapter  III.,  on  Sensations  of  Taste  and 
Smell,  Experiments  52-60. 

For  taste  experiments  the  following  general  apparatus 
is  necessary :  four  or  more  camel's-hair  pencils,  a  mirror, 


370       LABORATORY  COURSE  IN  PSYCHOLOGY. 

and  a  battery.  For  the  latter,  see  the  section  on  General 
Apparatus. 

Of  special  apparatus :  a  little  essence  of  clove,  materials 
for  making  the  various  test  solutions,  and  a  pair  of  small 
zinc  electrodes. 

The  Essence  of  Clove  is  for  the  demonstration  of  the 
habitual  associations  of  taste  and  smell  (Ex.  52).  The 
same  can  be  shown  even  more  conveniently  with  small 
Dennison  labels,  the  gum  of  which  is  flavored  with  sas- 
safras. They  may  be  had  at  almost  any  stationer's, 
and  being  dry  are  more  convenient  to  handle  than  the 
clove  solution.  For  proportions  of  oil  of  clove  and 
alcohol  in  this  essence,  see  the  "  essence  of  clove "  men- 
tioned below  among  the  materials  for  experiments  on 
smell. 

The  Solutions  should  be  made  of  two  strengths,  —  the 
stronger  for  testing  the  individual  papillae,  and  the  weaker 
for  finding  the  general  taste  areas  and  the  least  proportion 
tastable.  The  following  proportions  of  tastable  substances 
and  water  are  convenient.  Stronger  solutions  :  Sugar,  40  : 
100  ;  Quinine,  2  : 100  ;  Tartaric  Acid,  5  : 100  ;  Salt,  satu- 
rated solution.  Weaker  solutions  (for  which  the  water 
itself  should  be  without  taste)  :  Sugar,  5  : 100  ;  Quinine, 
2  : 100  000  ;  Tartaric  Acid,  5  : 1  000  ;  Salt,  2  : 100.  Spe- 
cial solutions  of  Sugar  for  Ex.  55  :  20  : 100,  18  : 100,  16  : 
100,  14  : 100,  12  : 100,  10  : 100. 

The  Zinc  Electrodes  can  be  made  by  soldering  bits  of 
sheet  zinc  an  inch  and  a  half  long  and  half  an  inch  wide 
to  pieces  of  ordinary  covered  copper  wire  of  convenient 
length. 

For  experiments  on  smell  the  following  will  be  required  : 
essence  of  clove,  an.  olfactometer,  camphor  gum.  yellow 
wax,  a  dozen  small  wide-mouthed  bottles. 

The  Essence  of  Clove  is  made  by  adding  one  part  of  oil  of 


SUGGESTIONS   ON  APPARATUS.  371 

cloves  to  fifteen  parts  of  alcohol,1  and  may  be  diluted  with 
water,  itself  odorless,  to  make  the  solutions  required  in 
Ex.  57  a.  For  that  experiment  dilutions  of  the  essence  that 
will  give  the  following  proportions  of  oil  of  cloves  will  be 
convenient :  1 :  50  000  ;  1 :  100  000  ;  1  :  200  000  ;  1  :  300 
000  ;  1 :  400  000 ;  1 :  500  000. 

A  number  of  Olfactometers  of  different  form  have  been 
suggested  by  different  experimenters.  The  olfactometer  of 
Zwaardemaker  is  so 
simple  in  construc- 
tion that  it  may  be 
made  in  the  labora- 
tory. It  will  be 
most  convenient  if 
made  double,  as 
shown  in  the  ac- 
companying cut. 

The  instrument 
consists  of  a  light 
wooden  screen,  say 
six  inches  square, 
provided  with  a  han- 
dle below  for  easier 

holding.  Through  this  screen,  a  little  below  the  middle,  a 
hole  an  inch  and  a  half  in  diameter  is  bored,  and  fitted  with 
a  large  cork.  The  cork  in  turn  is  pierced  with  two  holes 
side  by  side,  an  inch  apart,  and  of  such  size  as  to  fit  tight 
upon  the  glass  tubes  next  to  be  mentioned,  i.e.,  about  7mm. 
The  glass  tubes  should  be  long  enough  to  leave  10  cm.  free 
behind  the  screen,  and  about  3  cm.  free  in  front.  The  front 
ends  are  bent  upward  at  right  angles  for  insertion  in  the 

1  Whether  this  essence  is  of  the  same  strength  as  that  used  by  Lombroso  and 
Ottolenghi  in  their  experiments,  to  which  reference  is  made  after  Ex.  57,  the 
writer  does  not  know. 


372       LABORATORY  COURSE  IN  PSYCHOLOGY. 

nostrils.  The  odorous  substances  are  applied  in  the  form 
of  tubes  that  slide  over  the  glass  tubes  behind  the  screen. 
The  simplest  and  best  for  persons  of  normal  keenness  of 
smell  are  said  to  be  pieces  of  red  rubber  tubing  10  cm. 
long,  and  of  such  bore  as  just  to  slide  freely  over  the  glass 
tubes  (8mm.).  These  pieces  of  rubber  tubing  should 
themselves  be  slipped  into  pieces  of  tight-fitting  glass 
tubing,  so  as  to  prevent  the  spread  of  the  odor  from  their 
outer  surface.  For  Ex.  60  another  odor-tube,  this  time  of 
yellow  wax,  will  be  needed.  This  can  be  made  by  placing 
a  glass  tube  (of  the  size  of  the  air-tubes  used  in  the  olfac- 
tometer)  inside  a  tube  such  as  is  used  to  cover  the  rubber 
odor-tubes,  and  filling  the  space  between  them  with  melted 
wax,  and  afterward  withdrawing  the  inner  tube. 

The  assumption  upon  which  the  instrument  is  con- 
structed is  that  the  intensity  of  the  odor  .varies  directly  as 
the  surface  of  odorous  substance  exposed.  When  the  odor- 
tubes  are  slipped  upon  the  glass  tubes  of  the  olfactometer, 
and  pushed  back  until  their  ends  are  flush  with  those  of 
the  glass  tubes,  the  air  inhaled  through  the  latter  contains 
few  or  no  odorous  particles,  because  no  odorous  surface  is 
exposed.  When,  however,  the  odor-tubes  are  pulled  a 
little  away  from  the  screen,  so  that  they  extend  over  the 
ends  of  the  glass  tubes,  they  expose  the  odorous  surface  in- 
side them  to  the  current  of  air  inhaled.  The  strength  of 
the  odor  is  proportional  to  the  length  of  odor-tube  that 
extends  beyond  the  glass  tube.  The  length  of  odor-tube 
corresponding  to  a  just  observable  odor  will,  of  course, 
differ  with  different  tubes,  from  person  to  person,  and  with 
the  temperature ;  but  tubes  of  red  rubber  are  reported  to 
give  satisfactory  results,  both  as  to  original  intensity,  and 
the  constancy  with  which  they  keep  their  odor  through 
considerable  periods  of  time.  The  length  of  red  rubber 
odor-tube  required  by  Zwaardemaker  himself  for  a  just 


SUGGESTIONS   ON  APPARATUS.  373 

observable  odor  at  18°  C.  is  7  mm.  In  use  the  upward 
turned  end  of  one  of  the  glass  tubes  is  inserted  in  the  for- 
ward part  of  the  nostril,  and  the  subject  draws  his  breath 
in  the  way  most  natural  to  him  in  smelling ;  the  proportion 
of  odorous  particles  is  greater,  however,  when  the  current 
of  air  is  slow  than  when  it  is  rapid.  The  inside  of  the 
glass  air-tubes  may  need  to  be  cleansed  of  adhering  odorous 
particles  from  time  to  time.  Zwaardemaker's  olfactom- 
eter  is  also  made  in  a  more  perfect  form,  having  porous 
earthenware  cylinders  which  will  absorb  odorous  substances 
for  testing.  On  this  and  other  related  matters  see  his 
"  Physiologic  des  Geruchs,"  Leipzig,  1895.  An  olfactom- 
eter  on  somewhat  the  same  principle,  but  of  different 
form,  made  after  the  design  of  Dr.  Scripture,  is  also  to  be 
had. 

Apparatus   for   Chapter   IV.,    on    Sensations    of   Hearing, 
Experiments  61-103. 

Of  general  apparatus  the  following  will  be  required :  a 
small  package  of  absorbent  cotton,  three  yards  of  rubber 
tubing  (one-quarter  inch  outside  measurement),  a  pint 
bottle,  and  an  ordinary  clock. 

Of  special  apparatus  :  a  sound  pendulum,  a  pendulum 
carrying  a  small  tuning-fork,  a  rubber  hammer  for  striking 
tuning-forks,  low-pitched  forks  (or  Appunn's  lamella),  sev- 
eral forks  on  resonance  boxes,  a  set  of  mistuned  forks 
for  just  noticeable  difference  in  pitch,  various  small  tun- 
ing-forks, a  sonometer,  a  set  of  resonators,  two  or  more 
piston  whistles,  Galton  whistle  or  steel  cylinders  for 
highest  audible  tones,  two  or  more  bottle  whistles,  appa- 
ratus for  blowing  hydrogen  bubbles,  and  a  "  snapper 
sounder." 

The  Round  Pendulum  represented  on  the  following  page 
was  made  with  slight  modifications  from  the  instrument 


374      LABORATORY  COURSE  IN  PSYCHOLOGY. 

described  by  Kampfe,  and  pictured  also  in  Wundt  ("  Phys- 
iologischen  Psychologic,"  4te  Aufl.  I.,  361). l 


The  sound  is  produced  by  the  stroke  of  the  balls  at  the 
end  of  the  pendulum  rods  against  an  ebony  block,  and  is 
assumed  to  vary  in  intensity  in  direct  proportion  to  the 
height  of  the  fall,  or  to  the  square  of  the  sine  of  half  the 
arc  through  which  the  pendulum  swings.2  The  following 
table  is  calculated,  after  that  of  Kampfe  (p.  534),  to  show 
the  proportional  intensity  of  sounds  produced  by  falls  be- 
tween 30°  and  50°,  the  sound  at  40°  being  taken  as  unity. 

In  use  the  pendulum  rods  need  to  be  wrapped  with 
something  to  prevent  the  short  tone  that  they  give  after 
the  blow,  which  interferes  with  the  judgment  of  the  sounds. 
Kampfe  and  Wundt  mention  wrapping  with  felt,  and 


1  Though  the  instrument  here  appears  with  two  pendulums  and  two  arcs, 
one  with  a  single  pendulum  and  arc,  was  preferred  by  Kampfe,  and  would  be  both 
cheaper  and  better. 

2  For  the  justification  of  this  assumption,  see   Kampfe,    Wun (It's  PJiilos. 
Studien,  VIII.,  1892-93,  526  ff. 


SUGGESTIONS   ON  APPARATUS. 


375 


Kampf  e  found  it  necessary  to  wrap  the  free  end  of  the  arc 
with  the  same  material.  A  winding  of  small  rubber 
tubing,  as  shown  in  the  cut,  seems  tolerably  deadening. 
The  keys  on  either  side  of  the  ebony  block  are  covered 
with  felt,  and  were  intended  to  catch  the  pendulum  as  it 
returns  after  its  first  recoil,  and  so  prevent  repetitions  of 
the  sound.  The  same  can  be  done,  however,  as  conven- 
iently with  the  hand. 

Table  of  relative  intensities  of  sounds  when  the  sound  pendulum 
falls  through  any  angle  between  30°  and  50°,  the  sound  at  40° 
being  taken  as  unity. 


ANGLE. 

INTENSITY. 

ANGLE. 

INTENSITY. 

ANGLE. 

INTENSITY. 

30° 

0.57 

37° 

0.86 

440 

1.20 

31 

0.61 

38 

0.91 

45 

1.25 

32 

0.65 

39 

0.95 

46 

1.31 

33 

0.69 

40 

1.00 

47 

1.36 

34 

0.73 

41 

1.05 

48 

1.41 

35 

0.77 

42 

1.10 

49 

1.47 

36 

0.82 

43 

1.15 

50 

1.53 

For  demonstrational  purposes  the  pendulums  might  also 
be  released  from  the  fingers.     For  more  exact  tests  a  me- 
chanical release  is  preferable.      That 
attached  to  the   Clark  instrument  is 
pictured  in  the  accompanying  cut. 

The  release  is  shown  as  it  would 
appear  if  looked  at  from  a  point  about 
half-way  up  the  central  column  of  the 
instrument.  The  rod  of  the  pendu- 
lum rests  against  the  bent  portion  at 
the  left  end  of  the  crossbar.  The  lat- 
ter turns  easily  about  the  screw  by  which  it  is  fastened  to 
the  clamp,  by  which,  in  turn,  the  whole  is  fastened  to  the 
arc  of  the  instrument.  The  curved  piece,  standing  vertical 


376      LABORATORY  COURSE  IN  PSYCHOLOGY. 

in  the  cut,  is  provided  with  a  little  hook  at  its  lower  end, 
which  holds  the  crossbar  till  it  is  desired  to  release  the 
pendulum,  when  by  a  slight  upward  pressure  (that  is,  by  a 
movement  toward  the  observer  when  the  release  is  seen  as 
in  the  cut),  the  hook  is  drawn  away,  the  crossbar  turns, 
and  the  pendulum  falls.  The  clamp  is  held  in  place  on 
the  arc  by  a  set-screw  below.  It  is  essential,  of  course, 
that  the  release  work  as  easily  and  with  as  little  noise  as 
possible. 

Though  the  instrument  is  one  that  probably  would  be 
bought  ready  made,  the  following  dimensions  given  by 
Kampfe  may  be  convenient  to  some :  base-board,  of  oak, 
45  cm.  long,  15  cm.  broad,  3  cm.  thick;  central  column 
(steel),  33  cm.  high,  average  diameter,  2  cm. ;  cross-piece 
at  the  top  of  the  column,  8.5  cm.  long  and  1  cm.  in  diam- 
eter ;  length  of  pendulum  rods  (wood),  30  cm. ;  diameter  of 
balls  of  pendulum  (hard  rubber),  3  cm.  ;  striking-block 
(ebony),  7  cm.  long,  5  cm.  broad,  and  6  cm.  high.  This  is 
glued  to  the  base  instead  of  being  fastened  with  screws ; 
and  the  hole  in  it  is  made  so  large  that  it  does  not  touch 
the  central  column,  in  order  to  secure  as  uniform  and  un- 
complicated sounds  as  possible.  The  pendulums  swing  on 
points.  The  arcs  in  Kampfe's  instrument  .were  divided 
to  single  degrees,  and  in  some  parts  apparently  even  to 
tenths  of  a  degree.  For  general  use  single  degrees  would 
certainly  be  fine  enough.  The  whole  instrument  is  sup- 
ported upon  thick  pieces  of  felt  to  prevent  any  possible 
resonance  of  the  table  on  which  it  is  used.1 

When  the  instrument  is  to  be  used,  it  should  be  so  ad- 

1  In  the  instrument  in  the  Clark  laboratory,  the  chief  differences  are  in  the 
shape  of  the  ehony  block,  which  is  a  complete  parallelepiped,  and  the  conse- 
quent change  in  the  length  of  the  upper  cross-piece,  and  of  the  separation  of 
the  short  posts  from  which  the  arcs  spring.  The  central  column  is  also  a  little 
taller  and  the  pendulums  a  little  longer.  The  arcs  are  divided  at  intervals  of 
5  and  10  degrees,  the  intermediate  degrees  having  to  be  judged  by  eye. 


SUGGESTIONS   ON  APPARATUS. 


377 


justed  that  the  pendulums  swing  freely  and  the  balls,  when 
at  rest,  just  touch  the  ebony  block. 

Pendulum  for  Carrying  a  Small  Tuning-fork.  —  Any 
convenient  pendulum  can  be  used  for  this  purpose.  That 
described  below,  though  rude  in  construction,  has  proved 
useful  for  this  and  other  purposes. 

The  general  plan  of  the  instrument  is  shown  in  the  cuts 
below.  A  shows  the  face  view,  and  B  the  top  board  by 


,.e4Nj 


J) 


itself;  C  and  D  show,  on  different  scales,  the  screws  on 
which  the  pendulum  swings  and  the  sockets  that  receive 
them.  The  rod  of  the  pendulum  is  of  wood,  three  and 
a  half  feet  long,  three  inches  wide,  and  half  an  inch  thick. 
The  bobs  are  composed  of  several  six-inch  disks  of  sheet 
lead,  superposed  and  fastened  to  the  rod  with  a  single  screw 
through  the  centre.  The  screw  of  the  upper  disk  has  a  milled 
head,  so  that  it  may  be  more  easily  removed  for  altering  the 
position  of  that  bob.  The  lower  bob  is  fixed  permanently 
with  its  centre  seven  inches  from  the  end  of  the  rod.  The 
position  of  the  upper  bob,  shown  in  A,  makes  its  centre 


878      LABORATORY  COURSE  IN  PSYCHOLOGY. 

come  four  and  three-quarter  inches  from  the  upper  end. 
These  distances  are,  however,  of-  no  particular  significance, 
and  may  be  varied  as  convenient.  With  these  dimensions, 
and  bobs  weighing  about  three  pounds  each,  the  pendulum 
traverses  its  arc  once  in  something  less  than  1.5  seconds. 
By  placing  the  upper  bob  very  low  on  its  part  of  the  rod, 
the  time  may  be  quickened  to  one  second  or  under.1 

The  supporting-frame  is  made  of  wood  seven-eighths 
of  an  inch  thick,  and  has  the  following  dimensions  :  base 
board  36  X  8  inches,  uprights  24  X  4  inches,  top  board 
34  x  6  inches.2  The  top  board  is  pierced  with  an  elliptical 
hole  (8  X  2J  inches)  as  shown  in  Fig.  B.  The  pendulum 
swings  on  the  points  of  two  screws  passing  through  hard- 
wood brackets  fastened  on  opposite  sides  of  the  pendulum 
rod  (Fig.  C).  These  are  3|  inches  long  on  the  side  next 
the  pendulum  rod,  2f  inches  on  that  perpendicular  to  it, 
and  |  of  an  inch  thick.  The  lower  edge  of  the  brackets  is 
25^  inches  from  the  lower  end  of  the  pendulum. 

The  screws  are  ordinary  steel  wood -screws  about  2| 
inches  long,  filed  down  to  a  sharp  point  at  the  lower  end, 
and  extending  about  an  inch  below  the  brackets.  The 
points  stand  2i  inches  outward  from  the  pendulum  rod 
on  either  side.  It  is  important  that  these  should  go 
straight  through  the  brackets,  and  that  the  line  connecting 
their  points  should  be  perpendicular  to  the  rod  of  the  pen- 
dulum. The  points  rest  in  little  sockets  of  brass  and  glass, 
as  shown  in  section  in  Fig.  Z>.  The  upper  part  is  a  strip 
of  sheet  brass,  pierced  with  a  conical  hole  about  an  eighth 


1  The  instrument  may  be  given  a  very  fine  gradation  of  rates  by  having  the 
upper  bob  so  attached  as  to  be  moved  up  and  down  with  a  screw.    For  details 
of  such  an  instrument,  giving  very  slow  rates,  however,  see  Bowditch  and  War- 
ren, The  Knee-jerk  and  Its  Physiological  Modifications,  Journal  of  Physiology, 
XL,  1890,  29  if. 

2  This  frame  was  made  in  the  first  instance  for  another  purpose,  and  some 
of  its  dimensions  can  be  changed  with  advantage. 


SUGGESTIONS   ON  APPARATUS.  379 

of  an  inch  in  diameter  at  the  top  and  something  over  a 
thirty-second  at  the  bottom.  Between  the  brass  strip  and 
the  top  board  of  the  frame  (thus  closing  the  bottom  of 
this  conical  hole)  is  held  a  bit  of  smooth  glass,  a  piece  of 
a  broken  microscope  slide,  which  furnishes  the  actual  sup- 
port for  the  screw-point  mentioned  above.  These  sockets 
must,  of  course,  be  so  placed  that  they  do  not  hinder  the 
swinging  of  the  pendulum,  while  they  still  keep  it  in  its 
place. 

On  the  back  of  the  pendulum  rod,  a  little  below  the 
point  of  support,  a  couple  of  binding-posts,  such  as  are 
used  for  electrical  connections,  are  fastened.  The  holes 
in  these  posts  have  been  enlarged  to  about  three-sixteenths 
of  an  inch.  A  bit  of  steel  rod  is  passed  through  the  holes, 
and  on  it  a  small  brass  weight  is  fastened,  which  can  be 
shifted  to  one  side  or  the  other  to  compensate  any  ine- 
quality in  the  large  weights,  and  make  the  rest-position  of 
the  rod  come  at  the  middle  of  the  base-board. 

The  small  tuning-fork  (an  ordinary  one)  is  attached  to 
the  pendulum  by  thrusting  its  stem  into  a  suitable  hole 
near  the  lower  end  of  the  rod.  The  fork  must  be  so  placed 
that  the  tines  vibrate  lengthwise  of  the  pendulum,  not 
crosswise  of  it. 

A  Rubber  Hammer  is  easily  made  by  mounting  a  large 
rubber  cork  on  a  wooden  handle. 

Apparatus  for  HlgJiest  Audible  Tones.  —  Simple  appa- 
ratus for  this  purpose  is  all  more  or  less  unsatisfactory. 
The  most  convenient  instrument  is  probably  the  Galton 
whistle,  which  can  be  had  in  different  forms  and  at  various 
prices,  from  about  five  dollars  upward.1  The  instrument 
is  a  minute  whistle  or  organ-pipe,  blown  by  pressure  upon 
a  rubber  bulb,  and  capable  of  change  of  length  by  means 
of  a  plunger  which  closes  the  other  end  of  the  tube.  The 

1  Galtou,  Inquiries  into  Human  Faculty,  London,  1883,  p.  375  ff. 


380      LABORATORY  COURSE  IN  PSYCHOLOGY. 

precise  tone  given  with  any  position  of  the  plunger  is  diffi- 
cult to  fix,  however,  and  varies  with  the  pressure  of  the 
air  with  which  the  whistle  is  blown,  so  that  tests  with  it, 
at  the  best,  are  very  rough.  All  that  the  instrument  can 
safely  show,  except  when  handled  with  elaborate  precau- 
tions, is  the  general  character  of  very  high  tones,  and  the 
fact  that  some  persons  can  still  hear  tones  that  others  can- 
not. The  instruments  also  differ  among  themselves,  and 
some  are  said  to  produce  a  good  tone  with  settings  at  which 
others  give  one  much  obscured  by  the  rushing  sound  of 
the  air. 

The  following  table  of  values  is  given  by  the  Cambridge 
Scientific  Instrument  Company  of  Cambridge,  England, 
from  calculations  based  on  experiments,  for  an  instrument 
of  very  small  bore  (about  0.7  mm.)  made  by  them ;  but  it 
is  hardly  likely  that  they  themselves  would  contend  for  a 
high  degree  of  certainty  in  the  values  given,  especially  at 
the  upper  end  of  the  scale. 


LENGTH  OF  THE 
WHISTLE  IN  MM. 

NUMBER  OF 
VIBRATIONS. 

LENGTH  OF  THE 
WHISTLE  IN  MM. 

NUMBER  OF 
VIBRATIONS. 

1.0 

42,500 

3.0 

21,250 

1.2 

38,600 

3.5 

18,890 

1.4 

35,400 

4.0 

17,000 

1.5 

34,000 

5.0 

14,170 

1.6 

32,700 

6.0 

12,140 

1.8 

30,360 

7.0 

10,630 

2.0 

28,330 

8.0 

9,440 

2.5 

24,290 

9.0 

8,500 

10.0 

7,720 

The  following  values  are  from  results  found  by  Stumpf 
and  Meyer 1  in  actual  tests  with  a  Galton  whistle  of  a  form 


1  Schwiiigimgszahlbestiiiimungen  Me  sehr  lichen  Touen,  Wkilemcmn* s  An- 
nalen,  LX1.,  1897,  700-779. 


SUGGESTIONS   ON  APPARATUS. 


381 


devised  by  Professor  Edelmann  of  Munich.  The  bore  of 
this  instrument  was  4  mm.,  and  it  was  blown  with  com- 
pressed air. 


NUMBER  OF 
VIBRATIONS. 

LENGTH  OF  THE 
WHISTLE  IN  MM. 

NUMBER  OF 
VIBRATIONS. 

LENGTH  OF  THE 
WHISTLE   IN  MM. 

4,000 

19.6 

10,000 

6.6 

5,000 

15.2 

11,000 

5.7 

6,000 

12.4 

12,000 

4.8 

7,000 

10.4 

13,000 

4.25 

8,000 

8.85 

13,600 

3.9 

9,000 

7.5 

14,000 

3.7 

The  steel  cylinders,  sometimes  used  for  demonstrating 
tones  of  very  high  pitch,  are  to  be  had  from  dealers  in 
physical  instruments  generally ;  but  of  the  accuracy  with 
which  they  may  give  the  pitches  assigned  to  them,  the 
writer  is  ignorant. 

Apparatus  for  the  Lower  Limit  of  Pitch  is  also  some- 
what unsatisfactory.  A  good  instrument  for  demonstra- 
tion is  said  to  be  Appunn's  lamella,1  —  a  weighted  tongue 
of  steel  giving  rates  from  4  to  24  per  second.  A  thin  strip 
of  steel,  loaded  at  one  end  and  clamped  in  a  vise,  could  per- 
haps be  used,  and  its  rate  determined  afterward  by  making 
it  trace  on  a  smoked  glass  plate  or  otherwise.  In  any  case 
the  tones  thus  produced  are  very  weak,  but  will  serve  in 
Ex.  68,  where  the  intention  is  rather  the  demonstration 
of  the  character  of  these  very  low  tones  than  an  exact 
determination  of  the  limit  of  audibility. 
•  Tuning-forks.  —  The  tuning-forks  needed  for  the  experi- 
ments of  this  chapter  are  a  set  of  large  forks  on  reso- 
nance boxes,  including  the  following :  c,  cf  (two  of  this 
pitch),  c";  a  set  of  mistimed  forks  for  just  observable 


1  Anton  Appuun,  Niirnbergerstrasse,  12,  Hanau  a.  M.,  Germany. 


382       LABORATORY  COURSE  IN  PSYCHOLOGY. 

difference  in  pitch  "(or  a  pair  of  forks  one  of  which  is 
provided  with  running  weights)  ;  ordinary  forks  giving  a' 
and  c",  and  one  giving  c"f.  It  would  also  be  convenient 
to  have  a  set  of  large  forks  on  resonance  boxes,  giving  all 
the  tones  of  the  octave  from  c'  to  c",  but  this  is  not  neces- 
sary. 

The  Large  Forks  on  Resonance  Boxes  will  probably  be 
purchased.  The  others  can  be  made  without  very  great 
trouble  from  the  forks  sold  in  the  music-stores. 

The  Set  of  Mistuned  Forks  for  just  observable  difference 
in  pitch  may  be  prepared  from  such  forks.  Select  half 
a  dozen  forks,  picking  out  those  with  prongs  as  nearly 
alike  as  possible  in  all  respects,  and  such  as  sustain  their 
tone  well.  Take  one  of  these  as  a  standard,  and  tune  the 
next  sharp  by  about  one  beat  per  second,  the  next  by  two, 
the  next  by  three,  and  so  on.  The  forks  are  tuned  sharp 
by  filing  at  the  ends  of  the  prongs,  either  so  as  to  shorten 
them,  or  on  the  inside  near  the  ends  so  as  to  make  the  ends 
lighter.  Care  should  be  taken  to  keep  the  prongs  equal 
if  they  are  so  at  the  start,  or  to  correct  them  if  unequal. 
If  too  much  is  taken  off,  so  that  the  pitch  of  the  fork  needs 
to  be  reduced  again,  file  in  the  crotch  of  the  fork,  or  on  the 
prongs  near  the  crotch,  preferably  the  former.  Having 
tuned  the  forks  as  near  as  convenient  to  the  pitches  re- 
quired, mark  them  in  some  way  so  that  they  can  again  be 
recognized,  and  lay  them  aside  for  more  exact  counting 
later.  For  this  later  counting  prepare  a  resonance  bottle 
(see  below),  and  holding  both  forks  over  it  at  once,  count 
the  beats  for  ten  seconds  if  possible,  counting  the  first  beat 
"  nought."  Eepeat  the  counting  several  times,  but  not  at 
the  same  sitting,  and  take  the  average  of  the  results. 
Beats  from  2  to  4  per  second  are  best  for  counting,  and 
forks  beating  faster  or  slower  than  this  can  be  determined 
indirectly,  and  the  rates  of  all  checked  by  counts  of  various 


SUGGESTIONS   ON  APPARATUS.  383 

combinations.1  The  little  riders  mentioned  in  Ex.  71  may 
be  made  by  cutting  off  quarter-inch  bits  from  rubber  tub- 
ing that  will  fit  tight  upon  the  prongs  of  the  fork,  or  by 
using  little  spring  clips  of  wire. 

Forks  giving  a"  and  c'fr,  an  octave  higher  than  the 
ordinary  af  and  c"  forks,  can  be  made  by  cutting  off  the 
latter,  and  tuning  by  beats  with  other  a'  and  c"  forks. 
Fork  a",  for  example,  will  beat  with  a',  and  the  tuning 
is  to  be  continued  till  the  beating  disappears.  The  stems 
of  the  forks  may  in  this  case  be  pressed  against  a  table 
top  or  sounding-board,  to  strengthen  the  partial  tones  on 
which  the  beating  in  large  part  depends. 

For  the  Resonance  Bottle  mentioned  above,  any  bottle 
may  be  used,  and  can  be  tuned  to  the  right  pitch  by  pour- 
ing in  water,  which  raises  the  pitch ;  or,  if  the  mouth  is 
wide,  by  partially  covering  it  with  a  card,  which  lowers  the 
pitch.  A  six-ounce  wide-mouthed  bottle  is  about  right  for 
thg  c"  forks,  and  can  be  tuned  for  the  ans  by  partially 
covering  the  mouth  with  a  card,  the  proper  amount  of  cov- 
ering being  found  by  trial.  When  the  amount  is  once 
found,  the  card  may  be  fastened  in  position  with  wax. 
For  picture  and  description  of  such  bottles,  see  Mayer, 
"Sound,"  pp.  102  f. 

The  Sonometer  is  simply  a  long  flat  box  with  a  very 
thin  top,  which  serves  as  a  sounding-board  for  the  strings 
that  are  stretched  over  it.  One  can  be  had  from  the  phys- 
ical instrument-makers  at  prices  from  about  five  dollars 

1  The  following  are  the  directions  for  very  exact  counting  given  by  Ellis, 
Helmholtz's  Sensations  of  Tone,  2d  ed.,  London,  1885,  p.  443  f.  :  "  Count  on  one 
day  the  beats  between  forks  1  and  2, 3  and  4, 5  and  6,  etc.,  so  that  the  same  fork  is 
not  used  for  two  counts  on  the  same  day.  Excite  by  striking  with  a  soft  ball  of 
fine  flannel  wound  round  the  end  of  a  piece  of  whalebone,  as  a  bow  is  not  con- 
venient unless  the  forks  are  tightly  fixed.  Each  blow  or  bowing  heats,  and 
hence  flattens,  and  this  tells  if  the  experiments  on  any  one  fork  are  long  con- 
tinued. Count  each  set  of  beats  for  40  seconds  if  possible,  and  many  times 
over,  registering  the  temperature  and  the  beats,  and  taking  the  mean." 


384       LABOEATOET  COURSE  IN  PSYCHOLOGY. 

upward,  or  can  be  made  by  a  carpenter.  For  dimensions 
and  directions  for  making,  see  Mayer,  "  Sound,'7  pp.  129  f. 
For  many  experiments  any  stringed  instrument,  of  which 
many  cheap  forms  are  to  be  found  in  the  music-stores, 
would  do,  or  even  a  brass  wire  stretched  across  a  table  top. 
The  Resonators  will  probably  be  purchased.  The  best 
are  spherical.  Those  made  by  Konig,  the  celebrated 
acoustical  instrument-maker  of  Paris,  may  be  had  of  the 
physical  instrument  dealers,  but  are  expensive.  Cheaper 
conical  resonators  are  made  by  Appunn,  but  with  these  the 
writer  has  had  no  experience.  For  more  refined  apparatus 
for  many  of  the  experiments  of  this  chapter,  the  catalogues 
of  these  makers  should  be  consulted. 

The  Piston  Whistles  mentioned  in  the  text  could,  a  few 
years  ago,  be  purchased  in  the  toy-stores,  but  probably  are 
not  now  in  the  market.     Substitutes  for  them  can  easily 
be  made,  however,  by  notching  a  piece  of  small  brass  tubing 
of  smooth  bore  exactly  as  a  willow  twig  is  notched  in  mak- 
ing a  whistle,  and  fit- 
ting it  with  a  bit  of 
steel  rod  as  a  plunger. 
1  The  mouth  end  of  the 
whistle  should  not  be 
cut  slanting,  as  in  a 
willow     whistle,    but 
left  square.     A  three- 
or  four-inch  piece  of 
rubber    tubing    in  ay 
then    be    drawn    over 
this  end  for  a  mouth- 
piece.    Two  such  pis- 
ton whistles  may  take 
the  place  of  the  bottle  whistles. 

The   Bottle    Wltistles  are   even   simpler  in   manufacture 


SUGGESTIONS   ON  APPARATUS.  885 

than  the  piston  whistles.  They  are  made  by  fastening  a 
piece  of  rubber  tubing  to  the  lip  and  neck  of  a  bottle,  as 
in  the  cut,  or  better  still,  by  splitting  the  tube  a  little  way 
so  that  the  upper  half  may  extend  an  eighth  or  three-six- 
teenths of  an  inch  over  the  lip ;  but  care  must  be  taken 
that  it  does  not  project  too  far.  Bottles  of  wide  lip  give 
good  results.  For  a  similar  but  more  permanent  construc- 
tion, see  Helmholtz's  "  Sensations  of  Tone,"  p.  60.  For 
use  in  these  experiments,  prescription  vials  of  ounce  size 
will  be  convenient. 

On  Apparatus  for  Making  Hydrogen  consult  a  work  on 
chemistry.  Some  means  of  mixing  the  hydrogen  with  air, 
and  a  glass  tube  for  blowing  the  bubbles,  will  also  be  neces- 
sary. If  bubbles  can  be  blown  of  mixed  oxygen  and  hydro- 
gen instead  of  hydrogen  and  air,  the  effect  will  probably  be 
more  striking. 

The  "Snapper  Sounder"  or  "  telegraph  snapper,"  is  to  be 
had  of  dealers  in  telegraphic  supplies  at  from  about  thirty 
to  seventy-five  cents.     That  shown  in 
the  margin  is  of  the  lower  priced  sort. 

In  a  number  of  cases  experiments 
have  been  given  for  the  piano  or  parlor 
organ.  It  is  assumed,  of  course,  that 
these  will  be  borrowed.  Sometimes  the  specially  tuned 
Harmonical,  designed  by  Ellis  to  illustrate  the  theories  of 
Helmholtz  (see  description  of  the  instrument  in  his  trans- 
lation of  Helmholtz's  "  Sensations  of  Tone,"  pp.  466-469, 
also  17,  22,  and  168),  would  be  better.  This  instrument  is 
made  by  Messrs.  Moore  and  Moore,  104-105  Bishopsgate 
Street  Within,  London,  E.  0.,  at  between  forty  and  fifty  dol- 
lars. For  the  proper  tuning  of  the,  instrument,  however,  a 
special  set  of  nineteen  forks  is  necessary. 


386        LABORATORY   COURSE  IN  PSYCHOLOGY. 


Apparatus  for  Chapters  V.,  VI.,  and  VII.,  on  Vision, 
Experiments  104-233. 

Of  general  apparatus  the  following  will  be  required  :  a 
candle ;  two  pasteboard  tubes  about  an  inch  and  a  half  in 
diameter  and  a  foot  long ;  a  bit  of  straight  wire  six  or 
eight  inches  long ;  three  flat  buttons  ;  any  convenient  set 
of  pictures  (a  set  cut  from  the  illustrated  magazines  and 
mounted  on  cards  will  answer  every  purpose) ;  pieces  of 
plain  and  colored  glass,  about  5x8  inches  in  size  —  two  or 
three  pieces  of  plain,  one  piece  red,  one  piece  green,  three 
pieces  blue,  one  piece  each  of  any  other  convenient  colors ; 
white  tissue  paper,  colored  and  gray  papers,  black  and 
white  cardboard  ;  kindergarten  materials  (weaving  strips, 
rings,  and  dots) ;  and  colored  gelatine  in  the  principal 
colors  (including  violet  and  purple),  which  may  be  sub- 
stituted in  many  cases  for  the  colored  glass. 

Colored  and  gray  papers  can  be  had  from  firms  dealing 
in  materials  for  color-teaching  in  the  schools.  A  long 
series  of  gray  papers,  particularly  full  in  very  dark  grays, 
is  furnished  by  Rudolph  Kothe  of  Prag.  The  black  card- 
board should  be  dull  finished,  not  shiny.  Colored  cardboard 
is  also  to  be  had  from  the  printers  and  stationers,  and  is 
very  convenient  in  making  diagrams  requiring  colored 
backgrounds.  The  colored  gelatine  can  be  had  of  some 
of  the  dealers  in  physical  instruments  or  of  those  who 
furnish  stereopticon  supplies. 

Of  materials  for  special  experiments  the  following  will 
be  required :  a  pink-eyed  rabbit,  with  conveniences  for 
etherizing  and  removing  eyes,  and  a  little  modeling-clay ; 
three  or  four  inches  of  small  platinum  wire ;  a  little  chrome- 
alum,  though  this  may  be  omitted  if  purple  or  violet  gela- 
tine is  at  hand.  If  the  chrome  alum  is  to  be  used,  -make  a 
saturated  solution  in  water,  filter,  and  put  into  a  flat-sided, 


SUGGESTIONS   ON  APPARATUS.  387 

clear  glass  bottle.  Dilute,  if  necessary,  till  the  yellow 
spot  can  be  observed  as  described  in  the  experiment. 

Of  special  apparatus  there  will  be  needed  :  a  double 
convex  lens  of  short  focus  (a  reading-glass  will  answer)  ;  a 
double  concave  lens,  e.g.,  a  spectacle  lens ;  an  ordinary 
glass  prism  of  60°  angle  ;  a  prism  of  10-20°  angle,  to  be 
had  of  dealers  in  oculists'  supplies ;  a  campimeter ;  a  bat- 
tery and  electrodes  for  optical  stimulation ;  an  induction 
coil  and  small  Geissler  tube  ;  a  dark  box  with  photographic 
shutter  (the  latter  serves  very  conveniently  in  many  cases 
as  a  substitute  for  the  induction  coil  and  Geissler  tube)  ;  a 
color-wheel  and  a  number  of  special  disks ;  Holmgren's 
wools ;  a  spectroscope ;  a  metronome ;  a  reflection  color- 
mixer  ;  a  double  refraction  prism  ;  a  binocular  color-mixer  ; 
a  G-alton  bar  ;  a  krypteon  ;  medallions  and  specially  colored 
masks ;  a  frame  of  parallel  threads  as  described  in  Ex. 
196  c;  a  concave  mirror  ;  an  ordinary  stereoscope  (not  very 
necessary)  ;  a  haploscope ;  Martius-Matzdorff's  set  of  dia- 
grams ;  a  Wheatstone  stereoscope,  convertible  to  a  tele- 
stereoscope  ;  a  pseudoscope ;  a  zoetrope  ;  and  a  number  of 
special  diagrams  for  the  experiment  of  the  "  fluttering 
heart." 

The  lenses,  prisms,  and  several  other  pieces  will,  of 
course,  be  purchased,  and  need  110  further  comment. 

The  Campimeter  is  simply  a  large  but  thin  wooden 
plane  with  some  sort  of  support  for  the  head  fixed  before 
it.  That  in  the  Clark  laboratory,  which  is  shown  in  the 
cut  below,  has  a  surface  of  3  X  4  feet,  and  is  half  an  inch 
thick ;  a  plane  2x4  feet  would,  however,  answer  every 
purpose.  * 

The  plane  should  be  so  made  that  it  can  be  clamped 
to  the  table  top.  The  head-rest  is  8  X  9|-  inches  in  size, 
strengthened  by  a  two-inch  piece  across  the  back  at  the 
bottom,  and  mounted  on  the  top  of  a  steel  rod,  which  is 


388       LABORATORY   COURSE  IN  PSYCHOLOGY.. 

clamped  to  the  table  edge  with  one  of  White's  universal 
clamps  (see  section  on  General  Apparatus).  A  simpler 
head-rest  on  the  same  principle  can  easily  be  made  by  sub- 
stituting a  fixed  wooden  upright  for  the  adjustable  steel 


rod.  The  first  form  of  head-rest  is  not  absolutely  rigid, 
but  serves  well  enough  for  most  purposes.  If  absolute 
rigidity  is  required,  it  is  necessary  to  use  a  mouth-board 
like  those  described  by  Helmholtz  and  others.1 

A  more  perfect,  but  also  much  more  expensive,  instru- 
ment for  many  of  the  purposes  of  the  campimeter  is  the 
perimeter,  which  is  to  be  found  in  many  forms  in  the  cata- 
logues of  the  dealers  in  oculists'  instruments. 

Electrodes  for  Visual  Stimulation  can  be  made  by  solder- 
ing connecting  wires  to  plates  of  brass  or  zinc,  two  and 


1  For  pictures  of  mouth-boards,  see  Hering  in  Hermann's  Handbuch  der 
Physiologic,  III.,  i.,  pp.  440,  473,  and  478;  Helmholtz,  Physiologische  Optik 
2te  Aufl.,  p.  657  (p.  517  of  the  first  edition);  and  Aubert,  Physiologische  Optik, 
p.  647. 


SUGGESTIONS   ON  APPARATUS. 


389 


a  half  inches  wide  by  three  long,-  and  covering  them  with 
cloth.  Some  kind  of  a  key  for  opening  and  closing  the 
circuit,  and  a  commutator  for  changing  the  direction  of 
the  current,  are  helpful,  though  not  essential.  For  sugges- 
tions on  batteries,  see  the  remarks  on  General  Apparatus. 

A  Dark  Box  of  very  simple  construction  is  shown  in 
section  in  the  cut  in  the  margin.1 

A  box  seven  inches  square  (inside  measurement)  and 
eighteen  inches  high,  made  of  half -inch  stuff,  is  convenient. 
The  joints  may  be  made  light 

proof  by  covering  with  black          t  n 

cardboard  or  otherwise,  and 
the  box  provided  with  a  re- 
movable cover,  with  strips  in- 
side to  keep  it  in  place  and 
prevent  light  from  entering 
between  it  and  the  top  of  the 
box. 

A  little  larger  measurement 
than  seven  inches  would  be 
better,  but  that  size  has  the 
advantage  of  being  just  right 
for  the  use  of  cardboard  in 
lining  the  box.  The  ordinary 
black  and  white  cardboards  — 
come  22  x  28  inches  in  size, 

and  a  complete  lining  of  either  black  or  white,  can  easily 
be  made  by  cutting  a  strip  of  cardboard  of  the  required 
color  18  inches  wide,  marking  it  off  into  four  transverse 
strips,  each  18  X  7,  by  cutting  half  through  the  card 
from  the  back  with  the  point  of  a  knife,  and  folding  it 
up  into  shape.  When  an  object  or  diagram  placed  within 

i  Made  after  the  scheme  of  Helinholtz ;  see  his  Physiologische  Optik, 
2te  Aufl.,  p.  710  (p.  567  of  the  first  edition). 


390       LABORATORY  COURSE  IN  PSYCHOLOGY. 

the  box  is  to  be  seen  by-  instantaneous  illumination,  as  in 
Exs.  200  and  218  c,  it  is  well  to  line  the  box  throughout 
with  white.  When  very  complete  darkness  is  aimed  at,  as 
in  Exs.  136,  178,  and  226  &,  the  black  lining  should  of  course 
be  substituted.  The  height  of  the  box  is  unessential,  18 
inches  being  taken  smiply  because  it  brings  the  top  of  the 
box,  when  standing  on  the  table,  a  little  above  the  eyes  of 
a  seated  observer. 

Opposite  each  other,  in  the  front  and  back  of  the  box, 
two  horizontal  slots  should  be  cut,  about  3^  inches  long  by 
half  an  inch  wide ;  the  first  as  an  eye-hole,  the  second  to 
allow  a  little  light  to  enter  through  pin-holes  in  the  dia- 
grams when  points  of  fixation  are  required.  Kound  holes, 
suitably  spaced,  may  take  the  place  of  the  slots,  and  have 
the  advantage  of  being  easily  stopped  with  corks  if  desired. 
When  some  of  the  experiments  of  the  course  were  writ- 
ten, a  more  elaborate  box  was  in  mind.  Everything  essen- 
tial, however,  can  be  done  with  the  one  described,  and  the 
slight  changes  in  the  setting  of  the  experiments  will  offer 
no  difficulty. 

Instantaneous  illumination  can  be  secured  by  hanging  a 
Geissler  tube  in  the  box,  and  discharging  single  induction 
sparks  through  it.  A  convenient 
way  of  introducing  the  current 
and  hanging  the  tube  is  by  means 
of  double  binding-posts  having  a 
head  within  the  box  and  another 

without.  These  can  easily  be  made  of  a  couple  of  binding- 
posts  of  the  sort  having  separate  screws  for  attachment, 
by  sawing  off  the  head  of  the  screw,  passing  its  stein 
through  the  side  of  the  box,  and  screwing  on  another  post 
inside. 

Any  induction  coil  that  can  be  arranged  to  give  single 
sparks  can  be  used  with  the  Geissler  tube;  but  for  general 


SUGGESTIONS   ON  APPARATUS.  391 

laboratory  purposes,  apart  from  these  experiments,  the  coil 
should  be  one  in  which  the  primary  and  secondary  coils 
are  separable,  as  in  the  Du  Bois-Reymond  sliding  induc- 
tion coil,  well  known  in  the  physiological  laboratories. 

A  better  means  of  instantaneous  illumination,  in  some 
ways,  is  a  photographic  shutter  attached  to  the  top  or  side 
of  the  box.  It  is  a  little  difficult  to  cause  the  light  from 
the  shutter  to  fall  on  the  diagram,  but  it  may  be  accom- 
plished more  or  less  completely  by  properly  adjusting  the 
box  with  reference  to  the  source  of  illumination,  or  by 
placing  a  mirror  inside  the  box  to  direct  the  light  upon 
the  diagram. 

The  Color-Mixer  or  Color-Wheel  is  to  be  had  in  many 
forms  from  the  different  makers,  at  prices  ranging  from 
five  dollars  upward.  In  selecting  an  instrument,  it  is  im- 
portant to  see  that  it  is  easily  capable  of  sufficient  speed  to 
give  a  steady  mixture  with  a  disk  composed  of  one  part  of 
black  and  one  of  white  ;  that  it  runs  so  smoothly  that  the 
disk  does  not  flutter ;  and  that  it  is  provided  with  means 
for  ready  determination  of  the  proportions  of  the  differ- 
ent colors  combined,  e.g.,  by  a  protractor,  or  by  graduated 
disks.  A  little  electric  motor  serves  admirably  in  many 
cases,  if  batteries  or  other  means  of  driving  it  are  available. 
Many  of  the  experiments  can  be  made  with  the  color  tops 
sold  as  toys,  or  with  the  very  simple  one  suggested  by  Dr. 
Bowditch  in  his  "  Hints  on  Teaching  Physiology ;  "  to  wit, 
a  button-mould  fitted  with  a  peg,  and  spun  with  the  fingers. 
One  an  inch  and  three-quarters  in  diameter,  and  carrying 
disks  two  and  a  half  inches  in  diameter,  shows  the  con- 
trast effects  of  Ex.  152  d  as  elegantly  as  could  be  desired. 
The  disks  are  held  in  place  by  a  piece  of  rubber  tubing  of 
very  small  bore,  fitting  snugly  upon  the  stem,  and  twisted 
down  upon  the  disks  like  a  nut.  A  top  of  this  kind  with  a 
large  assortment  of  disks  is  now  to  be  had  from  the  physi- . 


392      LABORATORY  COURSE  IN  PSYCHOLOGY. 

cal  instrument  dealers  at  six  cents  each.  An  apparatus 
using  larger  disks,  and  operated  on  the  principle  of  a  boy's 
"buzzer,"  is  contained  in  Bradley's  Pseudoptical  Set  (as  is 
also  a  little  top  like  that  just  mentioned),  but  can  only 
be  used  with  disks  in  which  the  proportion  of  colors  is  per- 
manently fixed ;  for  the  motion  is  alternately  in  opposite 
directions. 

Several  dealers  furnish  disks  of  colored  paper  in  a  vari- 
ety of  colors  ready  cut.  These  are  a  great  convenience,  and 
at  times  almost  essential;  for  if  the  cutting  is  inexact, 
the  disks  will  appear  when  in  rotation  with  bothersome 
fringes  of  color. 

When  the  speed  of  rotation  is  sufficient,  disks  may  be 
slipped  together,  as  in  the  cut  below,  and  any  required  pro- 
portion of  colors  easily 
arranged.  If  the  rotation 
is  not  sufficiently  rapid, 
the  sectors  must  be  made 
smaller  and  more  numer- 
ous. This  is  not  much 
trouble  when  the  propor- 
tions of  color  are  to  re- 
main constant,  but  where 

adjustments  are  to  be  made,  the  multiplicity  of   sectors 
is  a  disadvantage. 

Besides  the  colored  paper  disks,  a  number  of  special  disks 
are  required  in  the  following  experiments :  — 

Ex.  124d.  (For  this  use  the  red  saturation  disk  of  Ex.  139.) 

128a.  Disk  bearing   a  number  of    equal  sectors   of  black  and 
white  ;  &,  spiral  disk  (see  cut  on  p.  117). 

139.  Set  of  saturation  disks. 

140.  Disks  for  least  change  of  intensity  (see  cut  on  p.  139). 

141.  Set  of  intensity  disks  (see  cut  on  p.  141). 
144.     Disks  for  Fechner's  colors  (see  cut  on  p.  395). 


SUGGESTIONS   ON  APPAllATUS.        '         393 

145.  Disk  with  the  same  proportions  of  black  and  white,  differ- 
ently arranged  in  concentric  rings  (see  cut,  p.  144)  ; 
also  a  disk  with  different  proportions  of  black  and  white 
similarly  arranged  ;  this  disk  also  shows  contrasting 
grays  (see  cut,  p.  145). 

152.  Contrast  disk  for  grays  (not  necessary  if  the  last  disk 
above  is  at  hand),  also  a  set  of  disks  for  color  contrasts 
(both  pictured  on  p.  158). 

160.     Disk  for  retinal  oscillation. 

163c.  Disk  for  contrast  with  induced  color  marked  off  by  heavy 
lines  of  demarcation. 

221a.  Disk  for  stereoscopy  with  moving  image  (see  cut  p.  295). 
6.  Disks  for  binocular  stroboscopy  (see  cut  p.  296). 

2286.  and  231.     Disk  with  radial  bands. 

228a.    Disks  for  monocular  stroboscopy  (see  cut  p.  296). 

2346.  Disks  for  demonstrating  Weber's  Law  (see  cuts  pp.  335  f. 
and  specifications  p.  412  f. ). 

Where  sets  of  disks  are  mentioned  in  the  above  list, 
single  disks  of  a  selected  color  may  be  substituted  in  case 
it  is  desired  to  limit  the  number.  Most  of  the  disks  have 
been  briefly  described  in  the  text,  but  a  few  additional 
particulars  may  be  helpful. 

In  planning  a  full  collection  of -disks,  it  is  well  to  fix 
on  a  standard  size  that  will  be  economical  of  cardboard. 
From  board  of  the  size  before  mentioned  can  be  cut  nine 
disks  nearly  eight  inches  in  diameter,  which  is  a  convenient 
size  for  individual  experimentation  or  demonstration  before 
small  groups  of  students.  The  same  sheets  allow  the  cut- 
ting of  two  disks  of  fifteen  inches  in  diameter.  When  no 
size  is  specified,  the  8-inch  disks  are  meant ;  when  large 
disks  are  mentioned,  the  15-inch,  unless  special  measure- 
ments are  given.  Most  of  the  cuts  in  the  text  show  the 
disks  in  the  form  required  for  moderate  rates  of  rotation, 
and  even  when  a  high  rate  is  attainable,  it  is  convenient 
to  have  these  permanent  disks  adjusted  for  slower  rates, 
because  of  the  steadier  motion. 


394       LABORATORY  COURSE  IN  PSYCHOLOGY. 

For  the  black-and-white  disk  of  Ex.  128  ay  that  used  in 
Ex.  152  d,  or  indeed  almost  any  one  of  the  black-and-white 
disks  of  other  experiments,  could 
probably  be  used.  If  a  special  disk 
is  required,  it  may  be  made  like 
that  in  the  margin. 

The  spiral  disk  for  Ex.  128  b 
should  be  of  the  larger  size.  It  is 
interesting  to  vary  the  experiment 
by  using  two  disks  at  the  same  time, 
one  large  and  one  small,  carrying 

spirals  of  opposite  direction,  the  small  one  put  on  in  front 
of  the  other.  To  draw  a  spiral  on  the  disk,  it  is  only  neces- 
sary to  fasten  a  pencil  in  a  thread  as  for  drawing  a  circle, 
and  then  to  have  the  thread  wind  about  a  post  of  suitable 
size  as  the  pencil  makes  its  circuits  ;  or,  better  perhaps,  to 
start  with  the  thread  wound  up,  and  to  trace  the  spiral  as 
the  thread  unwinds.  A  little  care  and  practice  will  pro- 
duce satisfactory  results.  The  writer  has  found  it  conve- 
nient in  drawing  spirals  to  fasten  the  cardboard  upon  a 
plank  or  table  top,  and  to  screw  into  the  plank  through  the 
cardboard  a  screw-post,  like  that  shown  at  half  size  in  the 
margin.  The  post  is  slightly  less  than  0.3  inches  in  diam- 
eter under  the  head,  and  thus  gives  a  spiral  with  the  corre- 
sponding parts  of  the  turns  almost  exactly  an 
inch  apart.  The  thread  is  put  through  a  hole 
in  the  head  of  the  post,  then  carried  up  and 
fastened  out  of  the  way  about  the  little  knob  on 
top.  In  order  to  keep  the  pencil  from  getting 
away  from  the  thread,  a  bit  of  sheet  brass  was  bored  with 
a  sixteenth-inch  hole,  and  tied  at  the  end  of  the  thread. 
This  is  shown  at  the  right  in  the  cut.  A  convenient 
width  for  the  black  line  of  the  spiral  is  seven-sixteenths 
of  an  inch,  and  for  the  white,  nine-sixteenths. 


SUGGESTIONS   ON  APPARATUS.  395 

The  saturation  disks  may  be  made  in  the  six  principal 
colors  (red  will  be  especially  needed),  and  will  be  made 
most  easily  by  pasting  colored  papers  upon  white  disks. 
The  color  must  be  given  a  leaf  shape,  something  like  the 
black  in  disk  A  on  p.  335,  but  narrower,  and  may,  with 
advantage  as  regards  speed,  be  distributed  upon  six  radii 
instead  of  three. 

The  disk  for  least  change  of  intensity  in  a  white  ground 
is  sufficiently  described  in  the  text,  p.  139. 

The  disks  for  showing  simultaneously  the  whole  range  of 
intensities  are  made  like  the  saturation  disks ;  but  the  col- 
ored paper  is  this  time  given  a  star  shape,  as  in  the  cut  on 
p.  141,  and  is  pasted  upon  black  disks  instead  of  white. 
While  not  required  for  the  experiment,  a  black  disk  carry- 
ing a  white  star  may  well  be  added  for  the  beauty  of  the 
grays  which  it  presents. 

Almost  any  of  the  black-and-white  disks  will  show 
Fechner's  colors  when  rotated  at  the  proper  speed.  For 
the  pierced  disk  mentioned,  Eood  made  use  of  four  radial 


slits  of  7°  each  ;  the  disk  should  be  of  black  cardboard. 
Since  Ex.  144  was  printed,  interest  in  these  colors  has 
been  aroused  by  the  publication  in  Nature  of  a  description 
of  a  special  top  that  exhibits  them  (vol.  li.,  1894-95,  see 
the  index  under  "top").  Fig.  A  above  shows  the  original 
form  described.  B  is  a  form  recommended  by  a  later  cor- 


396       LABORATORY   COURSE  IN  PSYCHOLOGY. 

respondent  in  Nature  (p.  510),  but  untried  by  the  present 
writer.  It  has  the  advantage  of  making  the  proportions  of 
black  and  white  variable,  which  is  said  to  be  an  important 
condition.  A  black  disk  and  a  white  one  carrying  a  spiral 
are  combined,  as  indicated  on  p.  392  above.  The  spiral 
line  in  the  cut  is  unfortunately  much  too  wide  —  a  line 
only  one-fifth  the  width  of  the  intervening  white  spaces, 
or  even  less,  is  recommended. 

The  disk  with  equal  quantities  of  black  and  white,  but 
differently  distributed,  and  that  for  different  proportions 
of  black  and  white  similarly  distributed,  will  be  sufficiently 
clear  from  the  cuts  on  pp.  144  f.  The  latter  is  repeated, 
except  as  to  the  number  of  gradations,  in  a  form  for  slower 
rotation,  in  the  cut  at  the  left  on  p.  158.  Both  would  be 
improved  as  contrast  disks  by  a  margin  of  white  on  the 
outer  edge. 

The  disks  for  color  contrast  may  be  prepared 'in  various 
colors,  e.g.,  in  the  four  principal  colors,  or  if  this  seems 
more  than  necessary,  in  green  alone  (see  cut  at  the  right 
on  p.  158).  The  color  may  be  conveniently  applied,  as  be- 
fore, in  the  form  of  colored  paper.  A  similar  disk,  carrying 
two  narrower  concentric  rings  for  contrast,  one  bordered  on 
both  sides  by  a  heavy  black  line  (e.g.,  1  mm.  broad),  is  used 
in  Ex.  153  c.  Both  will  probably  be  fully  understood  from 
the  description  in  the  text  (pp.  158  ff.). 

The  disk  for  retinal  oscillation  is  also  described  in  the 
text  (p.  168).  The  dimensions  there  given  are  not  essen- 
tial ;  smaller  disks  serve  as  well.  The  same  is  true  of  that 
for  stereoscopy  with  moving  figures  (pp.  294  f.).  The  only 
important  point  is  that  the  smaller  circle  should  not  be 
too  eccentric. 

The  disks  for  binocular  stroboscopy  (p.  296)  as  used  in 
the  Clark  laboratory  are  unnecessarily  large  (A  22  inches, 
B  16|  inches,  and  C  13  inches).  For  most  purposes  the 


SUGGESTIONS   ON  APPARATUS. 


397 


following  dimensions  would  be  better :  A  15  inches,  B  11 
inches,  C  7^  inches.  If  the  latter  are  chosen,  the  follow- 
ing would  be  about  right  for  the  dimensions  of  the  slits  : 
narrow  slits  in  A  1-*  inches  long,  with  their  outer  ends 
i  inch  from  the  edge  of  the  disk,  breadth  6° ;  the  curved 
sides  of  the  broader  openings  5  inches  and  3|  inches  re- 
spectively from  the  centre,  angular  breadth  about  40°.  The 
slits  in  B  should  be  the  same  distance  from  the  centre  as 
the  larger  openings  in  A,  and  in  angular  extent  the  same 
as  the  narrow  ones,  that  is,  6°. 

Disks  for  showing  the  ordinary  phenomena  of  strobos- 
copy  with  the  color-wheel  and  a  mirror  may  be  readily  pre- 


pared in  the  laboratory.  The  size  of  the  disks  will  of 
course  vary  with  the  apparatus  with  which  they  are  to  be 
used ;  the  following  dimensions  are  from  a  small  strobo- 
scope  turned  by  hand,  but  can  easily  be  changed  to  make 
disks  suitable  for  the  color-mixer. 

Diameter  of  disk  A  7|  inches,  diameter  of  circle  on  which 
the  openings  stand  6^  inches,  diameter  of  openings  y3F 
inch.  These  openings  were  cut  with  a  belt  punch ;  they 
may  be  replaced  by  radial  slits  if  more  convenient.  Di- 
ameter of  disk  B  6i  inches,  diameter  of  circle  passing 
through  the  centres  of  outline  circles  4|  inches,  diameter 


398      LABORATORY  COURSE  IN  PSYCHOLOGY. 

of  circle  passing  through  the  centres  of  outer  ring  of  black 
dots  3T9jv  inches  ;  same  for  inner  ring  of  black  dots  2  J  inches. 
The  outline  circles  are  just  under  $  of  an  inch  in  diameter, 
and  the  solid  black  dots  inside  them  just  under  1  of  an 
inch.  C  represents  the  disks  combined  for  use.  Unless 
both  sides  of  the  large  disk  are  black,  it  will  be  found 
better,  however,  to  turn  A  over  so  as  to  bring  the  black 
side  next  the  eye.  Such  a  pair  of  disks  shows  all  the 
essential  phenomena  of  the  stroboscope. 

The  disk  with  radial  bands  like  spokes,  mentioned  in 
Exs.  223  b  and  231,  should  be  of  the  larger  size  (15  inches), 
and  may  carry  a  dozen  bands  each  half  an  inch  wide. 

Colored  worsteds  for  making  Holmgren's  test  for  color- 
blindness can  be  had  from  the  dealers  in  oculists'  appara- 
tus, either  in  little  separate  skeins,  or  arranged  in  more  or 
less  elaborate  fashion  in  frames  or  holders  of  some  sort.  A 
simple  set  is  to  be  found  in  the  front  of  Galton's  "  Life 
History  Album." 

The  Spectroscope  used  in  the  Clark  laboratory  for  Exs. 
138  and  141  b  is  a  single  prism  instrument  of  no  very  great 
expense.  Any  instrument  which  shows  the  Fraunhofer 
lines  as  far  as  H  without  too  much  difficulty  would  proba- 
bly answer. 

The  Metronome  in  Ex.  145  is  such  as  is  used  by  musi- 
cians. If  the  pendulum  mentioned  above  (p.  377)  were 
arranged  with  an  electrical  contact  and  made  to  work  a 
telegraph  sounder,  it  would  answer  equally  well  for  this 
purpose  and  would  be  otherwise  useful.  The  simplest  sort 
of  electrical  contact  is  made  by  carrying  a  fine  wire  down 
the  pendulum  rod,  and  allowing  it  to  dip  into  a  little  glob- 
ule of  mercury  in  a  mercury  cup  on  the  base-board.  The 
upper  end  of  the  wire  twisted  into  an  open  spiral  may  be 
fastened  to  a  binding-post  set  into  the  front  of  the  frame 
near  the  point  of  support  of  the  pendulum,  and  will  not  in- 


SUGGESTIONS  ON  APPARATUS.  399 

terfere  very  much  with  the  swinging.  A  convenient  mer- 
cury cup  may  be  made  by  fastening  a  short  bit  of  glass 
tubing  with  wax  upon  the  top  of  the  upper  screw  of  a 
double  binding-post.  Such  a  cup  is  adjustable  in  height, 
and  has  in  the  lower  binding-screw  a  means  of  connection 
with  the  rest  of  the  circuit ;  but  care  must  be  taken  to 
remove  the  lacquer  from  the  top  of  the  upper  screw  so  as 
to  insure  good  contact  between  the  brass  and  the  mercury. 

The  construction  of  the  Reflection  Color-Mixer  is  proba- 
bly sufficiently  clear  from  the  description  and  cut  on  p.  150. 
The  same  is  true  of  the  Apparatus  for  Ragona  Scina's 
Experiment  (p.  156).  This  experiment  does  not,  however, 
require  a  special  piece  of  apparatus,  the  effect  being  very 
clear  when  the  diagrams  are  in  the  same  plane  and  the 
colored  glass  is  held  vertical  like  the  plain  glass  in  the 
reflection  color-mixer. 

For  the  Double  Refracting  Prism  in  Ex.  150  b  use  a  crys- 
tal of  Iceland  spar,  to  be  had  at  a  small  price  from  the  phys-   . 
ical  instrument  dealers.      The  requirement  of  achromatism 
made  in  the  last  line  of  the  experiment  is  unnecessary.1 

The  Binocular  Color-Mixer  mentioned  in  Exs.  156  a  and 
167  a  is  a  piece  of  apparatus  devised  by  Hering,  and  de- 
scribed by  him  in  the  Zeitschrift  fur  Psychologie,  I.,  1890, 
23-28.  It  is  made  by  Rothe  of  Prag,  in  neat  form  and  at 
a  low  price  ;  but  the  apparatus  is  simple,  and  any  carpenter 
can  make  of  wood  one  that  will  answer.  The  aim  in  Ex. 
156  is  to  secure  a  binocular  mixture  of  blue  and  red.  For 
this  purpose  blue  and  red  glasses  may  be  used  before  the 
eyes,  provided  that  a  good  deal  of  white  can  also  be  mixed 
in  with  the  color  of  the  glass.  This  is  accomplished  by 
letting  the  glasses  stand  at  an  angle,  and  reflect  on  their 

l  Rothe  makes  a  simultaneous  contrast  apparatus  after  a  plan  of  Bering's, 
in  which  such  prisms  are  used.  The  instrument  is  convenient  for  demonstrating 
color-mixing  by  this  method  as  well  as  for  contrast. 


400       LABORATORY  COURSE  IN  PSYCHOLOGY. 

upper  surface  the  images  of  suitably  placed  white  screens. 
The  quantity  of  white  light  is  regulated  by  the  position  of 
the  screens  with  reference  to  the  source  of  illumination,  and 
by  the  inclination  of  the  colored  glasses.  The  following 
cut  shows  the  arrangement  of  glasses  and  screens.  W-^  and 
Wz  are  the  screens,  R  and  B  the  red  and  blue  glasses,  W  a 
white  surface.  In  the  carpenter-made  instrument  in  the 
Clark  laboratory  the  following  plan  and  dimensions  were 
used :  in  the  middle  of  a  base-board,  30  inches  long  and 
12  inches  wide,  was  placed  another  board  1 2  inches  long  and 
10  inches  wide,  leaving  a  margin  of  an  inch  on  each  side, 

and  of  9  inches  at  the 
ends.  This  little  plat- 
form bears  a  piece  of 
white  cardboard  corre- 
sponding to  W  in  the 
diagram.  On  the  nearer 
edge  of  this  platform 
is  fastened  an  upright 
piece  15  inches  high 
and  3  inches  wide.  At 
its  upper  end,  on  the 

forward  side,  this  upright  carries  the  frames  that  hold  the 
glasses  R  and  B.  The  glasses  are  4  inches  square,  and 
are  framed  on  three  sides  only,  the  upper  edge  being 
left  free  so  that  the  glasses  may  come  close  to  the  eyes. 
(Glasses  2  inches  square,  or  even  less,  would  do  as  well 
or  better.)  The  frames  are  small  pieces  of  board  6  inches 
long  and  5  inches  wide,  with  a  square  piece  (three  and 
three-quarter  inches  on  the  side)  taken  from  the  middle 
of  their  upper  ends,  leaving  each  like  a  U  with  very 
square  corners  and  a  heavy  bottom.  Over  these  square 
holes  the  glasses  are  fastened.  The  frames  are  fastened 
with  a  single  screw  each  to  the  upright  before  mentioned, 


SUGGESTIONS   ON  APPARATUS.  401 

the  screws  entering  the  frames  about  an  inch  and  a  half 
below  the  free  edge  of  the  glass.  When  in  position,  the 
glasses  rise  about  three-quarters  of  an  inch  above  the  top 
of  the  post,  and  stand  like  the  sides  of  a  roof.  They 
do  not  quite  meet,  however,  but  leave  a  space  for  the  ob- 
server's nose  between  them  when  the  apparatus  is  in  use. 
The  screws  that  hold  the  frames  should  be  tight  enough  to 
keep  them  in  position,  but  not  so  tight  as  to  prevent  their 
turning  in  adjustment. 

The  side  screens  of  the  instrument  are  exactly  alike,  and 
the  description  of  one  will  do  for  both.  Each  is  a  piece  of 
half-inch  board  9  inches  wide  and  13£  inches  long.  This 
board  turns  midway  from  top  to  bottom  on  two  screws  put 
through  the  sides  of  a  light  frame  just  large  enough  to 
enclose  it.  The  frame  itself  is  fastened  to  a  broad  piece  of 
board,  which  forms  its  base  and  rests  in  turn  on  the  base- 
board of  the  instrument.  A  peg  in  the  middle  of  the  base 
of  the  frame,  fitting  into  a  hole  in  the  base  of  the  instru- 
ment, allows  the  rotation  of  the  frame  and  screen  about 
a  vertical  axis.  The  screen  is  thus  made  adjustable  in  two 
directions.  Its  face  is  covered  with  white  cardboard.  It 
is  highly  important  that  all  the  white  surfaces  be  without 
noticeable  spots,  and  the  colored  glasses  free  from  flaws. 

The  instrument  in  this  condition  will  serve  for  binocu- 
lar color-mixing.  For  simultaneous  contrast  by  Hering's 
binocular  method  (Ex.  156)  some  slight  additions  are  re- 
quired. On  the  front  of  the  upright,  and  six  inches  upward 
from  its  foot,  a  wire  about  three  and  a  half  inches  long  is 
set,  and  extends  forward  perpendicular  to  its  surface.  At 
the  end  of  the  wire  is  a  little  button  of  cork,  the  fixation 
point  K  in  the  diagram.  On  the  surface  W,  and  parallel 
to  this  wire,  is  pasted  a  strip  of  black  paper,  a  quarter  of 
an  inch  wide,  represented  by  S  in  the  diagram. 

An  instrument  constructed  on  this  plan  is  unnecessarily 


402       LABORATORY   COURSE  IN  PSYCHOLOGY. 

cumbrous,  and  would  probably  work  as  well  if  the  side 
screens  as  well  as  the  glasses  were  somewhat  reduced  in 
size.  The  rotation  of  the  screens  about  a  horizontal  axis  is 
also  hardly  worth  while. 

The  frame  of  Parallel  Threads,  mentioned  in  Exs.  196  c 
and  210  c,  is  probably  sufficiently  clear  from  the  text. 

The  Galton  Bar  can  be  had  ready  made  of  the  Cambridge 
Scientific  Instrument  Company,  under  the  name  of  "  Line 
Division  Testing  Apparatus."  As  furnished  by  this  com- 
pany it  consists  of  an  ebonite  strip  10  inches  long,  1  inch 
wide,  and  an  eighth  of  an  inch  thick.  On  the  back  of  this, 
and  extending  over  a  little  from  either  edge  upon  its  face, 
is  a  light  brass  slide.  The  parts  that  extend  over  upon 
the  face  carry  between  them,  crosswise  of  the  bar  and  close 
to  its  surface,  a  white  thread  which  divides  the  bar  into 
two  portions,  equal  or  unequal,  according  to  the  setting 
of  the  slide.  On  the  back  of  the  bar  is  a  fine  cross-line 
marking  the  middle  of  the  bar.  This  line  is  visible  through 
a  rectangular  opening  in  the  back  of  the  slide,  and  on  the 
edge  of  this  opening  a  scale  is  cut,  divided  to  tenths  of  an 
inch,  by  which  the  position  of  the  slide,  and  so  of  the 
above-mentioned  white  thread,  can  be  read  at  any  instant 
in  tenths  of  an  inch,  or,  since  the  bar  is  10  inches  long,  in 
percentage  of  the  total  length.  Besides  the  middle  line 
there  are  also  lines  at  one-third  and  one-fourth  the  length 
of  the  bar,  so  that  estimates  of  these  fractions  can  also  be 
made.  It  would  be  easy  of  course  to  make  such  a  bar 
from  any  rule  that  is  divided  on  one  side  only,  or  even 
from  a  straight  wooden  slat  on  which  a  strip  of  millimeter 
paper  has  been  pasted.1  For  a  somewhat  different  form  of 
the  Galton  bar,  see  the  apparatus  for  Chap.  VIII.,  below. 


1  It  should  be  noted,  however,  that  the  ordinary  metric  cross-section  paper 
does  not  measure  true  both  ways. 


SUGGESTIONS   ON  APPARATUS.  403 

The  instrument,  which  for  brevity  I  have  ventured  to 
call  a  Krypteon,  is  very  simple  in  principle,  —  nothing 
more,  indeed,  than  a  slanting  board  with  a  flap  hinged  at 
the  bottom  of  it.  It  is  roughly  pictured  in  the  accompany- 
ing cut. 

On  a  base-board  ab,  8  X  20  inches  in  size,  is  set  the  board 
cd,  6  X  18  inches,  inclined  backward  about  30°  from  the 
vertical.  At  the  ends  of  this  board  near  the  base  are 
fastened  short  brass  arms,  which  extend  forward  and  sup- 


port the  rod  ef.  They  are  of  such  length  as  to  bring  the 
centre  line  of  the  rod  f  of  an  inch  from  the  board  cd,  and 
|  from  ab.  The  rod  ef  is  provided  with  milled  heads  at 
the  ends,  so  that  it  may  be  rotated  easily  with  either  hand. 
The  rod  itself  is  composite,  being  made  of  pieces  of  half- 
inch  half-round  brass,  put  together  flat  side  to  flat  side,  to 
make  a  single  round  rod.  The  forward  half  of  the  rod  is 
in  three  pieces.  The  middle  piece  ik  is  held  in  place  by 
screws,  and  can  be  removed ;  the  end  pieces  are  soldered 


404       LABORATORY  COURSE  IN  PSYCHOLOGY. 

fast  to  the  back  half  of  the  rod.  This  arrangement  makes 
it  possible  to  clamp  securely  into  the  rod  the  cardboard  flap 
gh,  or  to  interchange  flaps  if  for  any  reason  this  is  desired. 
When  the  flap  is  in  position,  the  turning  of  the  rod  ef  will 
rapidly  cover  or  uncover  anything  placed  on  the  inclined 
surface  cd.  In  using  the  krypteon  in  Ex.  174,  a  narrow 
strip  of  wood  should  be  tacked  along  the  inclined  surface 
to  support  the  Galton  bar.  The  instrument  is  not  limited 
in  its  usefulness  to  this  single  experiment,  but  can  be  used 
for  experiments  with  after-images  and  successive  contrast, 
and  for  any  others  in  which  a  sudden  revealing  or  hiding 
of  an  object  is  desired.  The  instrument  can  hardly  be  re- 
garded as  a  necessity,  however,  and  a  simple  substitute  can 
readily  be  found  for  it. 

The  Concave  Mirror  used  in  Ex.  183  b  is  to  be  had  of  any 
optician  or  physical  instrument  dealer  at  small  cost. 

The  Mask  in  Ex.  184  can  be  purchased  at  a  toy-store, 
and  colored  as  required.  It  is  convenient  to  have  two 
masks  so  that  the  external  and  internal  aspects  may  be 
presented  simultaneously  —  in  this  case,  of  course,  only 
one  need  be  colored.  It  is  well  also  if  both  can  be  mounted 
in  some  way  for  more  ready  handling.  Medallions  in 
plaster,  about  four  inches  in  diameter,  may  be  had  in  many 
art  stores  at  a  very  low  price,  and  casts  of  these  in  op- 
posite relief  may  be  taken  in  the  laboratory  by  oiling  the 
surface  carefully,  surrounding  the  edge  with  a  strip  of 
paper,  and  pouring  on  plaster  of  Paris  mixed  to  about  the 
consistency  of  cream. 

An  Ordinary  Stereoscope  is  convenient  for  beginners  in 
binocular  experiments,  but  is  hardly  necessary,  especially 
if  a  haploscope  or  Wheatstone  stereoscope  is  included  in 
the  collection. 

The  Haploscope  is  nothing  but  a  simplified  stereoscope, 
and  has  the  sole  purpose  of  presenting  to  each  eye  a  field 


SUGGESTIONS   ON   APPARATUS. 


405 


invisible  to  the  other.1  It  is  hardly  necessary  if  a  Wheat- 
stone  stereoscope  is  at  hand.  In  the  cuts  below  is  shown 
a  somewhat  elaborate  form  of  it. 

In  the  figure  at  the  left  the  instrument  is  arranged  for 
parallel  vision,  in  that  at  the  right  for  crossed  vision.  In 
the  left  figure,  leaning  against  the  base,  are  also  shown  the 
special  movable  diagram-holders  for  Le  Conte  Stevens's 
Experiment  (Ex.  221  e).  It  would  probably  be  equally  con- 
venient and  much  less  expensive  to  purchase  two  ordinary 


stereoscopes,  remove  the  lenses,  and  reconstruct  one  into 
a  haploscope  for  parallel  vision  and  the  other  into  one  for 
crossed  vision,  than  to  make  one  after  this  model.  For 
this  reason  specifications  are  omitted,  but  a  few  measure- 
ments will  give  a  more  definite  idea  of  its  size  :  base-board, 
12i  x  5  X  If  inches ;  height  of  central  standard  to  centre 
of  axis  of  the  inclined  bar,  lOf  inches ;  inclined  bar, 


1  The  term  haploscope  is  used  by  Hering  for  a  piece  of  apparatus  much  more 
like  the  Wheatstone  stereoscope. 


406       LABORATORY   COURSE  IN  PSYCHOLOGY. 

18  X  2  X  If  inches  j  eye  tubes,  9i  inches  long,  \\  inch 
(inside)  diameter,  centres  of  tubes  2i  inches  apart. 

The  best  Stereoscopic  Diagrams  with  which  the  writer  is 
familiar  are  those  referred  to  in  the  text  as  Martius-Matz- 
dorff's  diagrams.  The  set  consists  of  36  diagrams  and  an 
explanatory  pamphlet,  and  is  published  by  Winckelmann 
und  Sohne,  Berlin,  under  the  title,  Die  interessantesten 
Erscheinungen  der  Stereoscopic. 

The  Wheatstone  stereoscope  is  very  simple  in  plan,  as  may 
be  seen  from  the  cut  below. 


e 

I 


i  "i 

66  / 


The  eyes  look  into  mirrors  set  at  right  angles  to  each 
other,  ab  and  ac,  and  see  the  reflected  images  of  diagrams 
placed  at  de  and  fg.  In  actual  construction,  however,  the 
instrument  is  varied  somewhat  from  this  very  simple  plan. 
Thus  for  experiments  with  diagrams  at  different  distances; 
it  is  desirable  to  have  de  and  fg  movable  to  and  from  the 
mirrors  ;  for  experiments  with  different  degrees  of  conver- 
gence, to  have  the  mirrors  and  diagrams  movable  together 
about  centres  lying  below  the  mirrors  (or  better  still  about 
centres  lying  in  the  same  vertical  lines  as  the  centres  of 
rotation  of  the  eyes)  ;  for  use  as  a  telestereoscope  the  dia- 
gram-holders de  and  fg  must  be  replaced  by  large  mirrors 
parallel  to  the  small  ones  (see  plan  on  p.  279)  ;  and  if 
Stevens's  experiment  is  included,  the  diagram-holders  must 
be  turnable  about  a  vertical  axis. 

The  following  cut  represents  a  large  wooden  apparatus 


SUGGESTIONS   ON  APPARATUS.  407 

built  up,  a  little  at  a  time,  in  the  Clark  laboratory.  The 
base  of  the  instrument  is  a  bench  8  inches  high,  12  inches 
wide,  and  4  feet  long.  On  this  rest  arms  which  carry  at 
their  inner  ends  little  columns,  at  the  upper  ends  of  which 
are  attached  the  mirrors  corresponding  to  ab  and  ac  of  the 
plan,  and  at  their  outer  ends  the  diagram-holders,  the  left 
one  of  which  is  turned  part  way  so  as  to  show  the  diagram 


in  place.  Back  of  the  bench  stand  the  large  mirrors  which 
take  the  place  of  the  diagram-holders  when  the  instrument 
is  used  as  a  telestereoscope.  Underneath  the  bench  is 
hung  a  drawer  in  which  the  diagrams  are  kept  when  not 
in  use. 

For  the  experiments  of  this  course,  however,  a  somewhat 
smaller  and  simpler  apparatus  would  be  not  only  more 
convenient,  but  in  many  ways  better ;  and  the  writer  there- 
fore offers  the  following  plans  instead  of  a  detailed  de- 


408       LABORATORY  COURSE  IN  PSYCHOLOGY. 

scription  of  this  instrument,  though  circumstances  have 
forbidden  their  execution  at  the  time  of  writing.1 

The  plan  shows  the  instrument  on  a  scale  of  about  one- 
tenth.  The  first  figure  shows  the  front  view ;  the  second, 
the  horizontal  plan ;  and  the  third,  one  of  the  diagram-hold- 
ers seen  from  the  station  of  the  mirrors. 

The  base  of  the  instrument  is  to  be  a  wooden  bench  ^0 
inches  long,  8  inches  wide,  and  the  same  in  height.  On 
the  top  of  this  bench  are  fastened  two  arms  made  of  nar- 
row strips  of  hard  wood  14|-  inches  long,  1^  inch  wide. 
and  half  an  inch  thick,  as  shown  in  the  first  and  second 
figures  of  the  plan.  The  adjacent  ends  of  these  arms  are 
rounded,  and  they  are  fastened  to  the  top  of  the  bench 
with  a  single  screw  each.  These  screws  serve  as  pivots 
about  which  the  arms  can  be  turned.  The  separation  of 
these  screws  should  be  2^  inches  from  centre  to  centre, 
and  they  should  be  set  two  inches  back  from  the  front  edge 
of  the  base-board.  Each  arm  carries  also  near  its  inner 
end  an  angular  bracket  which  supports  the  mirror.  These 
brackets  may  be  made  an  inch  thick,  and  8|  inches  in  height 
from  the  upper  surface  of  the  arm.  Their  extent  on  the 
arm  is  not  important,  provided  it  is  not  too  great,  say  3 
inches ;  and  they  should  be  cut  back,  as  shown  in  the  plan, 
enough  to  allow  the  insertion  of  the  pivot  screws.  They 
must  be  so  set  that  the  centre  line  of  the  mirror  surface 
stands  exactly  over  the  centre  of  the  pivot  screw.  The  up- 
permost inch  and  a  half  of  these  brackets  is  first  worked 
out  one  inch  square,  and  then  cut  away  on  the  diagonal  so  as 
to  receive  the  mirrors,  somewhat  as  shown  by  the  small  fig- 
ures just  above  the  brackets  in  the  first  figure  of  the  plan. 


1  For  plan  and  description  of  a  refined  and  somewhat  specialized  instrument 
of  this  kind,  see  Hillebrand,  Die  Stabilitat  der  Raumwerthe  auf  der  Netzhaut, 
Zeitschriftfilr  Psychologic,  V.,  1893,  p.  38;  see  also  Hering,  Hermann's  Hand  bitch 
der  Physiologic,  III.,  i.,  393  f. 


SUGGESTIONS   ON  APPARATUS. 


409 


410       LABORATORY  COURSE  IN  PSYCHOLOGY. 

The  surface  left  for  the  mirrors  is  thus  nearly  an  inch  and 
a  half  square. 

The  mirror  itself,  one  inch  square,  may  be  framed  in 
cardboard,  and  attached  by  very  small  screws  to  this  sur- 
face. A  cardboard  frame  can  be  built  up  as  follows :  from 
pasteboard  as  thick  as  the  mirror  glass  cut  a  piece  1| 
inch  square,  and  cut  in  its  centre  an  inch  square  hole  to 
receive  the  mirror.  Paste  on  the  back  of  this  a  1^  inch 
square  of  thin  cardboard,  put  the  mirror  into  its  place,  and 
paste  on  in  front  of  it  a  piece  of  black  cardboard  having 
a  |  inch  square  cut  in  its  centre. 

The  diagram-holders  are  of  simple  construction.  Each 
consists  of  a  sliding  block  four  inches  square  and  half  an 
inch  thick,  on  the  bottom  of  which  are  fastened  two  cleats 
(each  4x1$  inches),  leaving  a  space  between  them  just 
equal  to  the  width  of  the  arm.  On  this  sliding  base  rests 
another  block  of  the  same  dimensions.  The  two  are  held 
together  by  a  single  screw  put  up  from  below  through  the 
middle,  as  indicated  in  the  third  figure  of  the  plan,  where 
the  blocks  are  a  little  separated.  The  screw  must  fit  tight 
enough  in  the  lower  block  to  allow  the  upper  to  be  turned 
stiffly.  On  the  middle  of  the  upper  block  rises  the  upright 
of  the  diagram-holder  (11^  inches  high,  4  inches  wide,  and 
half  an  inch  thick).  Crosswise  of  this  are  fastened  two 
little  guides  of  wood  for  receiving  the  diagrams  (which  are 
themselves  5^  X -7  inches  in  size).  These  should  be  placed 
as  high  as  the  upright  will  allow.  The  upright  is  braced 
by  a  little  bracket  in  front,  as  shown  in  the  plans.  The 
mirrors  required,  when  the  instrument  is  to  be  used  as 
a  telestereoscope,  should  be  of  good  quality  and  8  X  15 
inches  in  the  clear.  They  should  be  framed  in  plain 
wooden  strips,  and  the  frames  fastened  to  the  backs  of 
the  upright  pieces  of  the  diagram-holders.  In  using  the 
telestereoscope  on  the  actual  landscape,  it  is  well  to  open 


SUGGESTIONS   CLV  APPARATUS.  411 

the  windows  so  as  to  avoid  the  distorting  effects  of  the 
ordinary  window  glass. 

The  Pseudoscope  in  the  Clark  laboratory  is  of  the  sort 
manufactured  by  Pellin  of  Paris.  The  instrument  is  con- 
venient, but  a  less  expensive  one  would  answer  every  pur- 
pose. Two  total  reflection  prisms  placed  at  the  proper 
distance  apart  on  a  block,  and  kept  in  place  with  wax, 
could  probably  be  used.  If  a  construction  of  more  per- 
manent character  is  undertaken,  it  should  allow  for  some 
rotation  of  the  prisms  about  their  vertical  axes,  and  for  an 
alteration  of  their  separation. 

Diagrams  for  the  Experiment  of  the  "  fluttering  Heart " 
(Ex.  230)  are  perhaps  sufficiently  described  in  the  text ; 
but  the  following  details  with  regard  to  the  set  in  the  Clark 
laboratory  may  not  come  amiss,  though  they  are  not  fur- 
nished as  beyond  improvement.  The  colored  papers  used 
were  mostly  those  supplied  by  the  Milton  Bradley  Company, 
and  reference  will  accordingly  be  made  to  their  standards 
throughout.  For  Ex.  230a  blue  rings  on  a  red  ground : 
blue,  kindergarten  rings  of  standard  blue ;  red,  about 
equal  to  orange-red.  For  red  rings  on  a  blue  ground : 
red,  Bradley's  red  kindergarten  rings ;  blue,  something 
between  blue  and  violet-blue,  but  darker  than  either ;  cross 
lines,  very  narrow  strips  of  white  paper.  For  Ex.  2306 
gray  figures  on  colored  ground  and  colored  figures  on  gray 
ground  :  gray  on  red,  gray  about  equal  to  that  given  by 
270°  black  cardboard  combined  with  90°  white  on  the 
color-mixer  ;  red  about  equal  to  orange-red.  Colored  fig- 
ures on  gray  ground,  rings  of  standard  orange ;  gray,  that 
used  for  the  cover  of  the  American  Journal  of  Psychology. 
In  the  second  paper  of  Szili,  mentioned  in  the  text  in  con- 
nection with  this  experiment,  will  be  found  specifications 
for  diagrams  made  from  the  Helmholtz  papers  (furnished 
by  Jung  of  Heidelberg)  and  commercial  gray  papers  of 


412       LABORATORY  COURSE  IN  PSYCHOLOGY. 

German  manufacture.    The  same  author  uses  grays  shaded 
to  the  proper  depth  with  a  lead-pencil. 

Apparatus  for  Chapter  VIII.,  on  Weber's  Law  and  the  Psy- 
chophysic  Methods,  Experiments  234-239. 

The  only  pieces  of  apparatus  requiring  special  mention 
in  this  case  are  the  disks  and  set  of  weights  used  for 
demonstrating  Weber's  law,  and  the  special  form  of  Gal- 
ton  bar  used  in  the  experiments  on  the  psychophysic 
methods. 

The  disks  for  Weber 's  law  are  described  in  the  text,  but 
the  following  details  of  the  specimens  in  the  Clark  labora- 
tory (see  Figs.  C  and  Z>,  p.  336)  may  be  helpful.  The 
disks  are  40  cm.  in  diameter  (though  15-inch  disks  would 
answer  just  as  well).  Beginning  at  the  centre,  the  black 
extends  unbroken  for  a  distance  of  2.5  cm.  At  that  point 
the  white  begins,  and  gradually  increases  till,  at  a  point 
16  cm.  further  out  (1.5  cm.  from  the  edge  of  the  disk),  it 
occupies  the  whole  360°.  The  black  is  distributed  on 
three  radii,  as  the  cuts  show;  and  the  curves  were  plotted 
according  to  calculations  made  for  the  angular  amount  of 
white  at  each  centimetre  from  the  point  at  which  the  white 
begins  to  the  point  at  which  it  occupies  the  whole  circum- 
ference. Since  the  black  is  not  absolutely  black,  but  re- 
flects a  small  amount  of  light,  an  allowance  is  made  for 
this  amount,  on  the  assumption  that  the  black  is  about 
one-fiftieth  as  bright  as  the  white  of  the  cardboard  from 
which  the  disk  is  cut.  The  table  on  page  413  gives  the 
angular  extent  of  white  at  the  indicated  distances  from 
the  centre. 

These  particular  disks  are  finished  by  a  narrow  black 
line  all  the  way  around  at  the  extreme  edge  of  the  disk, 
but  this  is  not  at  all  essential. 


SUGGESTIONS   ON  APPARATUS. 


418 


Dr.  Kirschmann  gives  general  formulae  for  such  calcula- 
tions in  the  article  cited  in  the  bibliography  of  Chap.  VIII. 
For  the  brightness  values  of  various  black  pigments  in  com- 
parison with  white  paper,  see  an  earlier  article  by  the  same 
author  (Wundt's  Phllos.  Studien,  V.,  1889,  300). 

Angular  Extent  of  White. 


DISTANCE 
FROM  THE 
CENTRE 
IN  CM. 

ARITH- 
METICAL 
SERIES, 
DISK   C. 

GEO- 
METRICAL 
SERIES, 

DISK  D. 

DISTANCE 
FROM  THE 
CENTRE 
IN   CM. 

ARITH- 
METICAL 
SERIES, 

DISK  C. 

GEO- 
METRICAL 
SERIES, 

DISK  D. 

2.5 

0.0° 

0.0° 

10.5 

180.0 

44.6 

3.5 

22.5 

2.0 

11.5 

202.5 

59.0 

4.5 

45.0 

4.6 

12.5 

225.0 

77.4 

5.5 

67.5 

8.0 

13.5 

247.5 

100.8 

6.5 

90.0 

12.2 

14.5 

270.0 

130.8 

7.5 

112.5 

17.6 

15.5 

292.5 

169.1 

8.5 

135.0 

24.5 

16.5 

315.0 

217.9 

9.5 

157.5 

33.3 

17.5 

337.5 

280.3 

18.5 

360.0 

360.0 

The  Weighted  Envelopes  used  in  Ex.  236  were  made  by 
loading  stout  manila  "  pay  envelopes,'7  4|  x  2£  inches  in 
size,  with  pieces  of  sheet  lead,  or,  in  the  case  of  lighter 
weights,  with  pieces  of  cardboard.  A  series  was  thus  pro- 
duced ranging  from  5  to  100  grams.  As  originally  planned, 
the  series  consisted  of  106  weights,  besides  the  light  and 
heavy  standards,  which  were  duplicates  of  the  lightest  and 
heaviest  weights  of  the  series.  The  differences  were  not 
uniform  throughout,  but  smaller  with  the  smaller  weights 
and  larger  with  the  larger :  5  to  10  grams,  26  weights,  dif- 
ference 0.2  gram ;  10.5  to  25  grams,  30  weights,  difference 
0.5  gram ;  26  to  50  grains,  25  weights,  difference  1  gram j 
52  to  100  grams,  25  weights,  difference  2  grams.  Some 
such  arrangement  of  the  differences  is  necessary  if  the 
heavier  classes  are  not  to  be  very  much  larger  numerically 


414       LABORATORY   COURSE  IN  PSYCHOLOGY. 

than  the  lighter.  As  actually  used,  the  series  consisted 
of  118  weights  besides  the  standards,  104  from  the  regular 
series,  and  14  others  of  weights  between  36  and  38  grams. 
In  all  of  the  envelopes,  even  the  lightest,  a  piece  of  thin 
cardboard  (twice  the  size  of  the  envelopes,  but  folded  once) 
was  placed  to  stiffen  them,  and  make  the  light  and  heavy 
ones  feel  a  little  more  alike.  The  envelopes  should  be 
lifted  vertically  between  the  thumb  and  finger ;  and  the 
lead,  when  it  does  not  fill  the  envelope,  should  lie  at  the 
lower  end.  For  this  particular  experiment  the  weighing 
does  not  need  to  be  extremely  exact ;  a  fair  approximation 
to  the  values  given  above  is  sufficient. 

The  form  of  Galton  bar  used  in  Exs.  238  and  239  is 
shown  at  about  half  size  in  the  following  cut. 


It  consists  of  a  narrow  steel  rule,  graduated  on  the  back 
in  hundredths  of  an  inch,  and  divided  on  the  face  by  a 
single  line  in  the  middle,  and  of  two  large  sliding-pieces, 
one  at  either  end.  The  instrument  was  made  of  two  of 
Messrs.  Brown  and  Sharp's  rule  depth  gauges.  All  the 
graduation  was  removed  from  one  side  of  the  rule,  the 
single  middle  line  put  in  its  place,  and  the  inner  ends  of 
the  two  heads,  or  sliding-pieces,  bevelled  down  to  the  sur- 
face of  the  rule.  Either  head  can  be  fixed  firmly  in  place 
by  means  of  the  screw  on  its  upper  surface.  The  rule  it- 
self is,  in  this  case,  only  six  inches  long,  which  is  incon- 


SUGGESTIONS   ON  APPARATUS.  4l5 

veniently  short ;  but  longer  rules  could  probably  be  obtained 
from  the  makers.  The  chief  advantage  of  an  instrument 
of  this  general  form  (and  more  convenient  variants  are 
quite  possible)  is  that  it  allows  the  setting  of  a  constant 
standard  extent  with  which  the  variable  extent  is  com- 
pared, —  a  point  of  little  consequence,  perhaps,  in  investi- 
gation, but  of  some  importance  in  the  exposition  of  the 
psychophysic  methods.  In  Exs.  238  and  239  standards 
were  chosen  that  left  little  or  none  of  the  ends  of  the  rule 
exposed  beyond  the  heads. 

General   Apparatus. 

Besides  the  apparatus  for  specific  purposes  already  con- 
sidered, a  certain  amount  of  general  apparatus,  used  more 
or  less  in  all  experiments,  is  required.  The  needs  of  those 
making  use  of  this  chapter  will  probably  differ  so  widely 
that  the  writer  contents  himself  with  enumerating  what 
is  likely  to  prove  useful  without  advising  as  to  style  or 
quantity.1 

First  is  to  be  mentioned  a  substantial  set  of  rods,  stands, 
clamps,  and  couplers.  They  may  be  had  of  any  dealer  in 
physical  or  chemical  apparatus,  but  vary  much  in  quality. 
Those  should  be  selected  which  are  well  enough  made  to 
be  firm  and  solid  when  combined  for  use.  A  combination 
that  will  wobble  when  set  up  is  of  no  satisfaction  whatever. 
A  few  universal  couplers  or  ball-joint  clamps  are  very  con- 
venient. A  variety  of  ball-joint  and  swivel  clamps  and 
couplers  manufactured  by  Otis  C.  White  of  Worcester, 
though  not  originally  intended  for  laboratory  use,  have 
given  eminent  satisfaction,  and  may  now  be  had  from  some 
of  the  physical  instrument  dealers.  One  of  the  ball-joint 

1  For  other  general  suggestions  on  laboratory  furnishing,  see  the  writer's 
paper,  "  Some  Practical  Suggestions  on  the  Equipment  of  a  Psychological 
Laboratory,"  American  Journal  of  Psychology,  vol.  v.,  1892-93,  429-438. 


416       LABORATORY  COURSE  IN  PSYCHOLOGY. 

table  clamps  with  rod  to-  fit  has  been  presupposed  in  the 
description  of  the  head-rest  of  the  campimeter  (p.  388). 
Besides  these,  a  number  of  ordinary  iron  clamps,  such  as 
are  to  be  had  at  the  hardware  stores,  are  useful  for  attach- 
ing pieces  of  apparatus  to  the  table. 

Electric  batteries  have  several  times  been  mentioned  or 
implied  in  previous  paragraphs.  Ex.  121  has  been  made 
in  the  Clark  laboratory  with  a  battery  of  four  Leclanche 
cells  of  the  "  gonda  "  pattern,  but  these  are  less  convenient 
for  other  purposes.  For  running  small  electric  motors, 
induction  coils,  and  the  like,  the  Edison-LaLande  battery 
(type  S  for  example)  has  been  recommended,  and  would 
serve  equally  well  for  any  of  the  experiments  of  this 
course,  though  it  has  not  been  used  for  them  by  the 
writer.  These,  however,  are  not  convenient  for  taking 
from  place  to  place.  For  the  latter  purpose,  the  familiar 
Grenet  battery  would  be  preferable. 

A  set  of  drawing-instruments  is  essential  if  disks  and 
diagrams  are  to  be  prepared  in  the  laboratory,  and  to  that 
may  be  added  india  ink,  brushes,  etc.  A  few  of  the  com- 
mon carpenters'  and  machinists'  tools  are  also  almost  in- 
dispensable. 

Delicate  balances  for  use  in  making  the  minimal  weights 
of  Ex.  22,  and  coarser  ones  for  the  cartridge  weights  for 
Exs.  24  and  34,  have  been  assumed  in  the  experiments 
above.  If  the  weights  are  not  bought  ready  made,  and 
the  stales  cannot  be  borrowed,  they  must  be  added  to  the 
required  apparatus.  The  same  is  true  of  measuring-vessels 
for  making  the  solutions  for  the  taste  and  smell  experi- 
ments. 

Additional  Apparatus  for  Alternate  Forms  of  Experiment. 

In  a  few  cases  alternate  forms  of  experiment  have  been 
mentioned  which  call  for  apparatus  not  included  in  the 


SUGGESTIONS   ON  APPARATUS.  417 

lists  considered.  A  few  words  about  these  pieces  may  be 
in  place. 

The  Antirrheoscope  mentioned  in  Ex.  128  d  is  the  same 
described  and  pictured  by  Bowditch  and  Hall  (Journal  of 
Physiology,  III.,  1880-82,  297-307)  and  by  James  in  his 
« Principles  of  Psychology,"  II.,  245. 

The  Apparatus  of  Dvorak  (Ex.  221  b)  is  described  in  his 
original  paper  (see  Bibliography  of  Chap.  VII.),  and  re- 
ferred to  more  fully  than  in  the  text  in  the  American  Jour- 
nal of  Psychology,  VI.,  1893-95,  575  ff. 

Some  form  of  Rotating  Drum  has  once  or  twice  been  men- 
tioned, and  the  instrument  has  a  wide  usefulness  in  experi- 
ments not  included  in  Part  I.  of  this  course.  The  best 
instruments  of  this  kind  (known  from  one  of  their  phys- 
iological applications  as  Kymographs)  are  all  very  expen- 
sive. Cheaper  substitutes  are  offered  by  various  makers 
in  this  country  and  abroad,  some  for  movement  by  hand, 
and  others  by  clockwork  or  small  motors.  In  selecting 
one,  care  should  be  taken  to  get  a  drum  that  runs  true  011 
its  axis,  and,  if  self-driven,  as  uniformly  as  possible. 

The  Pressure  Balance,  though  mentioned  in  the  experi- 
ments as  an  alternate  only,  is  convenient,  and  may  be  briefly 
described.  The  one  in  the  Clark  laboratory,  made  after 
the  suggestion  of  Professor  Jastrow,1  consists  of  a  medium- 
sized  Fairbanks'  letter-scale  provided  with  a  wooden  base 
and  hand  support,  and  a  lever  and  cam  for  removing  the 
pressure  from  the  finger.  The  general  construction  will  be 
clear  from  the  accompanying  cut.  In  the  larger  diagram, 
A  is  the  pan  of  the  scale,  B  its  arm,  C  the  lever,  and  D 
the  cam. 

The  tips  of  the  fingers  are  thrust  into  a  horizontal  open- 
ing in  the  left  upright  board  of  the  frame,  which  is  shown 

1  American  Journal  of  Psychology,  III.,  1890-91,  54  f. 


418       LABORATORY   COURSE  IN  PSYCHOLOGY. 

in  front  view  in  the  small  diagram  at  the  right.  In  the 
same  diagram  is  also  shown  the  hard  rubber  knob  on  the 
end  of  the  scale  arm,  which  presses  upward  against  the  fin- 
ger when  the  pressure  stimulus  is  being  applied.  This  is 
counterbalanced  by  a  bit  of  wire  attached  to  the  frame- 
work under  the  scale  pan,  or  by  a  piece  of  felt  laid  on  top 
of  the  pan.  The  finger-rest  and  the  end  of  the  scale  arm 
are  also  shown  in  section  in  the  small  diagram  at  the  left. 
The  stimulus  weights  are  placed  in  the  scale  pan  A,  and 


exert  an  upward  pressure  on  the  finger,  but  of  reduced 
amount  because  acting  upon  the  short  arm  of  the  balance 
—one-quarter  their  actual  weight  in  the  Clark  instrument. 
Depressing  the  lever  arm  of  the  cam  raises  the  latter,  and 
allows  the  knob  to  reach  and  press  upon  the  finger  ;  raising 
the  lever  arm  removes  the  pressure.  The  chief  advantages 
of  the  instrument  are  the  ease  and  precision  with  which 
the  stimuli  are  applied  and  removed,  and  the  unconstrained 
position  of  the  hand  of  the  subject. 


SUGGESTIONS   ON  APPARATUS.  419 


A  Minimal  List  of  Apparatus. 

It  is  hardly  possible  that  any  who  use  this  course  will 
wish  to  try  all  the  experiments  or  require  all  the  appa- 
ratus. Each  one  ought  to  examine  the  experiments  for 
himself,  choose  what  he  is  most  interested  in,  and  make 
up  an  apparatus  list  accordingly.  It  is  therefore  with 
some  doubt  as  to  whether  a  minimal  list  will  be  a  really 
helpful  thing  that  the  author  offers  the  following  as  seeming 
to  him  the  most  important  pieces :  weights  for  pressure 
and  lifting,  including  those  of  equal  weight  but  unequal 
size ;  a  sonometer ;  ten  or  a  dozen  ordinary  tuning-forks 
of  af  and  c"  pitches  for  special  tuning  (by  careful  filing,  a 
heavy  fork  of  of  pitch  may  be  reduced  to  c',  and  give  the 
octave  with  c")  ;  a  resonance  bottle  ;  a  pair  of  bottle  whis- 
tles ;  a  yard  of  small  rubber  tubing  ;  a  color-wheel,  colored 
papers,  black  and  white  cardboard  for  disks  and  diagrams  ; 
some  small  pieces  of  colored  gelatine  ;  a  60°  prism;  a  dark 
box  ;  a  Wheatstone  stereoscope  convertible  to  a  telestereo- 
scope ;  a  small  set  of  drawing-instruments ;  and  a  couple 
of  yards  of  metric  cross-section  paper. 

Such  a  list  includes  several  pieces  that  may  be  made  at 
home,  and  ought  not  to  cost  more  than  twenty-five  or  thirty 
dollars.  Makeshifts  would  often  have  to  be  used,  and  or- 
dinary things  about  the  room  pressed  into  service,  but  with 
it  a  considerable  number  of  experiments  could  be  made. 
A  large  number  of  visual  experiments  are  possible  with 
the  material  included  in  Bradley's  Pseudoptics  ;  and  if  even 
the  small  list  above  is  too  expensive,  that  may  be  substi- 
tuted for  the  color-wheel,  the  Wheatstone  stereoscope,  and 
most  of  the  materials  for  disks  and  diagrams. 


APPENDIX   I. 


The  Field  of  Regard  and  Listing's  Law. 

EXPERIMENT  172  requires  a  somewhat  fuller  understanding  of 
Listing's  law  than  can  be  gathered  from  Ex.  131  6,  where  the  sub- 
ject was  previously  treated.  It  has,  therefore,  seemed  best  to  attempt 
a  fuller  exposition  of  it  here. 

Listing's  law,  as  stated  by  Helmholtz,  is  as  follows  :  "  When  the 
line  of  regard  passes  from  the  primary  position  to  any  other  position, 
the  angle  of  torsion  of  the  eye  in  its  second  position  is  the  same  as  if 
the  eye  had  come  to  this  second  position  by  turning  about  a  fixed  axis 
perpendicular  both  to  the  first  and  the  second  position  of  the  line  of 
regard."  l  On  this  principle  rest  two  important  corollaries  :  1st,  in 
movements  from  the  primary  position  there  will  be  no  rotation  about 
the  line  of  regard  ;  2d,  in  movements  from  one  secondary  position 
to  another  there  will  be  such  rotation. 

THE   HEMISPHERICAL    FIELD    OF   REGARD. 

The  usual  way  of  putting  the  law  to  experimental  test  is  to  get  a 
strong  after-image  of  a  rectangular  cross  on  the  centre  of  the  retina, 
and  then  to  observe  the  changes  that  its  projected  image  undergoes 
as  the  eye  is  turned  to  one  point  and  another  of  the  field  of  regard. 
In  the  model  from  which  the  accompanying  illustration  is  taken,  an 
attempt  has  been  made  to  show  the  changes  that  such  an  after-image 
would  undergo  when  projected  upon  different  parts  of  a  hollow  hemi- 
spherical field.  The  primary  meridian  of  this  field  is  A  *  Z?,2  and 
other  meridians  are  shown  at  intervals  of  20°.  The  equator  of  the 
field  (that  is,  the  line  of  intersection  of  the  plane  of  regard  with 
the  hemispherical  field  of  regard  when  the  eyes  are  in  the  primary 
position)  is  C  *  D,  and  above  and  below  it  are  shown  parallels  at 

1  Helmholtz,  Phys.  Optik,  2te  Aufl.,  p.  623,  Ite  Aufl.,  p.  466 ;  Le  Conte,  Sight,  147. 

2  In  naming  the  curves  of  the  hemispherical  field,  the  asterisk  (*)  is  used  for 
the  central  cross  instead  of  a  letter. 

421 


422 


APPENDIX    I. 


intervals  as  before  of  20°.  The  eye  itself  is  supposed  to  be  at  tho 
centre  of  the  sphere,  i.e.,  in  the  plane  of  the  letters  JL,  If,  G,  JV, 
etc.,  and  at  the  centre  of  the  circle  that  they  mark. 

Let  us  first  illustrate  the  case  of  movements  from  the  primary 
position.  When  the  eye  is  in  its  primary  position,  it  is  directed  for- 
ward and  fixed  upon  the  central  eight-rayed  cross.  Imagine  that 
the  eye  takes  a  lasting  after-image  from  the  cross,  but  first  from  the 
horizontal  and  vertical  arms  only.  If  the  point  of  regard  is  elevated 


or  depressed  in  the  primary  meridian,  and  there  is  no  rotation  about 
the  line  of  regard,  the  vertical  bar  of  the  after-image  cross  will  still 
be  found  to  lie  in  the  meridian  ;  and  if  the  point  of  regard  be  carried 
to  the  right  or  left  in  the  e'quator  of  the  field,  the  horizontal  bar  will 
still  lie  in  the  equator.  This  is  shown  by  the  slender  crosses  40° 
from  the  centre  on  A  *  B  and  C  *  D.  The  axes  about  which  the 
eye  turns  are  evidently  in  the  plane  of  the  letters  A,  If,  6?,  JV,  etc., 
and  coincide  in  the  first  case  with  the  (imaginary)  diameter  C  D, 
and  in  the  second  with  the  (imaginary)  diameter  A  B.  Suppose 
now  that  the  after-image  has  been  taken  from  the  oblique  anus  of 


FIELD   OF  EEGAED  AND  LISTING'S  LAW.      423 

the  central  cross,  and  that  the  movement  of  the  eye  has  been  oblique 
to  the  right  and  upward,  and  to  the  left  and  downward  along  H  *  £, 
and  to  the  left  and  upward,  and  to  the  right  and  downward  along 
E  *  .F,  but  without  rotation  about  the  line  of  regard.  As  before, 
those  arms  of  the  cross  which  originally  coincided  with  these  lines 
will  be  found  to  coincide  with  them  after  the  movement,  as  shown 
by  the  corresponding  arms  of  the  slender  crosses  in  these  positions. 
The  axis  for  movements  in  G  *  H  lies  in  the  (imaginary)  diameter 
E  jP,  and  that  for  movements  in  E*  Fin  the  (imaginary)  diameter 
G  H.  For  any  intermediate  directions  of  movement,  the  axes  would 
have  a  corresponding  intermediate  position ;  but  in  all  cases  the  axes 
would  lie  in  the  plane  of  the  letters  A,  K,  £,  JV,  etc.,  perpendicular 
to  the  line  of  regard  both  before  and  after  its  movement. 

Since  these  after-images  are  always  projected  on  a  hemisphere, 
there  is  no  distortion  of  any  of  the  crosses  due  to  projection  on  an 
oblique  surface,  and  all  of  their  parts  maintain  among  themselves 
exactly  the  same  relations  that  exist  among  those  of  the  central 
cross.1  It  will  be  observed,  however,  that  in  the  oblique  positions 
the  arms  corresponding  to  the  vertical  of  the  central  cross  do  not 
quite  coincide  with  the  meridians  passing  through  the  centres  of  the 
crosses,  but  make  small  angles  with  them,  and  that  in  the  same 
way  the  arms  corresponding  to  the  horizontal  in  the  central  cross 
have  no  longer  the  same  direction  as  the  parallels  above  and  below 
them.  In  other  words,  the  vertical  and  horizontal  arms  appear  to 
have  rotated,  though  the  fact  that  the  oblique  arms  have  maintained 
their  coincidence  with  the  circles  E  *  F,  and  G  *  II  shows  that  the 
rotation  is  not  real,  but  as  Le  Conte  says,  "only  an  apparent  rota- 
tion consequent  up" on  reference  to  a  new  vertical  meridian  of  space." 
This  apparent  rotation  is  known  as  torsion.  The  rule  for  this  torsion 
is  as  follows  :  Movement  of  the  eyes  upward  and  to  the  right  gives 
torsion  to  the  right  ;  upward  and  to  the  left,  torsion  to  the  left ; 
downward  and  to  the  right,  torsion  to  the  left  ;  downward  and  to 
the  left,  torsion  to  the  right  —  all  of  which  can  easily  be  observed 
in  the  cut.  Movements  from  any  secondary  position  to  the  primary 
are  evidently  executed  about  the  same  axes  as  before,  but  in  the 
contrary  direction. 

It  remains  to  consider  movements  from  one  secondary  position  to 


1  This  is  true  of  the  model  after  which  the  cut  was  made,  but  is  not  true  of 
the  crosses  in  the  out  itself,  which  are  obviously  distorted  because  of  just  such  a 
projection.  This,  however,  does  not  affect  the  explanations  that  follow. 


424  APPENDIX  i. 

another.  Let  us  start  with  an  after-image  from  the  slender  cross 
on  C  *  -D,  40°  to  the  right  of  the  centre,  and  move  upward  along 
the  meridian.  The  vertical  arm  of  this  cross  coincides  with  the 
meridian  at  the  start.  When  we  reach  the  position  of  the  eight- 
rayed  cross,  however,  it  no  longer  does  so,  but  has  turned  slightly 
to  the  right  —  this  time  owing  to  a  true  rotation  of  the  eye  about  the 
line  of  regard,  and  not  to  reference  to  a  new  meridian.  The  amount 
of  rotation  is  small,  in  this  case  about  12°.  Movement  downward 
along  the  meridian  would  have  exactly  the  same  result,  except  that 
the  rotation  would  be  in  the  opposite  direction,  and  similar  rotations 
would  be  found  if  the  cross  40°  to  the  left  of  the  centre  on  C* D  had 
been  used  for  vertical  movements,  or  the  crosses  40°  above  and  below 
the  centre  on  A  *  B  had  been  used  for  right  or  left  movements. 

If  movements  from  secondary  positions  along  great  circles  pro- 
duce this  deviation  of  the  arm  of  the  cross  from  the  line  in  which 
it  moves,  are  there  any  lines  to  be  found  along  which  the  eye  may 
sweep  the  after-image  without  finding  such  a  deviation  ?  There  are 
such  lines,  and  four  of  them  are  shown  in  the  figure.  They  are  the 
arcs  I J,  EL,  M N,  and  O  P.  It  will  be  seen  that  these  are  drawn 
through  the  sloping  positions  of  the  arms  of  the  side  crosses  on  E*  F 
and  G  *  H ,  and  are  perpendicular  to  A  *  B  and  C  *  D  like  the  bars 
of  their  crosses.  These  are  the  Circles  of  Direction,  or  Right  Circles, 
of  Helmholtz  (Cercles  de  Direction,  Richtkreise1).  The  vertical 
circles  of  direction  have,  it  will  be  observed,  somewhat  greater 
curvature  than  the  meridians  through  the  same  points,  and  the 
horizontal  circles  of  direction  somewhat  less  than  the  parallels 
near  which  they  lie.  Along  these  circles  a  short  after-image  can  be 
moved  without  leaving  the  line,  a  peculiarity  in  which  they  resemble 
a  straight  line,  and  when  seen  with  the  eye  at  rest  under  proper 
conditions  they  actually  do  appear  straight.  These  circles  have  the 
further  peculiarity  that  they  all  pass  through  the  occipital  point,  a 
point  as  far  behind  the  eye  as  the  primary  point  of  regard  is  in  front 
of  it.  Both  of  these  properties  are  shared  also  by  all  the  great  cir- 
cles passing  through  the  primary  point  of  regard,  so  that  they  also 
are  circles  of  direction.  A  circle  of  this  kind,  great  or  small  as  the 
case  may  be,  can  be  passed  through  any  two  points  in  the  field  ; 
they  are  not  limited  to  those  shown  in  the  figure. 

The  mathematical  study  of  Listing's  law  shows  that  the  move- 
ment from  one  secondary  position  to  another  may,  like  those  from 

1  Op.  cit.,  pp.  C51  ff.,  GOO  if.  (493  if.,  548  f.). 


FIELD   OF  REGARD  AND  LISTING'S  LAW.      425 

the  primary  position,  be  conceived  as  rotations  about  fixed  axes  aK 
of  which  lie  in  a  plane  (though  in  this  case  the  plane  is  not  per- 
pendicular to  the  line  of  regard),  and  that  in  every  case  there  is  also 
a  line  about  which  there  is  no  rotation,  the  atropic  line,  though  this 
does  not  coincide  with  the  line  of  regard. 

THE    PLANE    FIELD    OF   REGARD. 

The  experimental  testing  of  Listing's  law  is  generally  carried 
out  with  the  plane,  instead  of  the  hemispherical,  field  of  regard, 
because  of  the  difficulty  of  providing  a  large  enough  hollow  hemi- 
sphere. But  this  has  the  disadvantage  of  adding  to  the  changes  in 
the  after-image  due  to  the  movements  of  the  eyes,  a  wholly  new  set 
of  distortions  due  to  the  projection  of  the  image  upon  an  oblique 
surface.  These  are  easily  seen  in  the  figure  for  the  plane  field. 

This  figure  is  a  gnomonic  projection  of  the  hemispherical  field 
upon  a  plane  tangent  to  it  at  the  middle  point  of  the  central  cross. 
On  this  plane  all  the  lines  of  the  hemispherical  field  are  represented 
exactly  as  their  shadows  would  be  cast  by  a  point  of  light  in  the 
place  of  the  eye,  i.e.,  in  the  centre  of  the  sphere.  The  meridians 
are  represented  by  vertical  straight  lines,  wider  and  wider  apart  as 
they  are  removed  from  the  primary  meridian  A  B.  The  parallels 
become  hyperbolas,  increasing  in  curvature  as  they  are  more  distant 
from  the  equator  of  the  field.  The  great  circles  through  the  primary 
point  of  regard  are  straight  lines  through  the  same  point.  The 
other  circles  of  direction  are  hyperbolas.  They  maintain  their  re- 
semblance to  straight  lines,  however,  in  so  far  as  concerns  a  short 
linear  after-image  moved  along  them,  and  are  called  by  Helmholtz 
the  right  lines  of  the  field  of  regard.  The  lettering  of  all  the  lines 
in  the  two  figures  is  the  same,  so  that  comparison  will  be  easy. 

The  distortion  of  the  crosses  on  A  B  and  C  D  is  easy  to  under- 
stand, and  also  the  oblique  bars  of  those  on  E  F  &nd  G  H  represented 
in  outline  in  the  figure.  The  arms  corresponding  to  the  vertical  and 
horizontal  arms  of  the  central  cross  —  represented  in  solid  black  in 
the  figure  —  require  a  little  explanation.  If  the  matter  were  one  of 
simple  projection,  without  torsion,  the  arm  corresponding  to  the 
vertical  ought  to  coincide  with  the  projection  of  the  meridian,  and 
that  corresponding  to  the  horizontal  ought  to  coincide  with  the  pro- 
jection of  a  line,  cutting  the  meridian  at  right  angles  in  the  hemi- 
spherical field,  i.e.,  with  the  projection  of  the  parallel  that  passes 
through  the  centre  of  the  cross,  —  the  dotted  lines  in  the  figure. 


426 


APPENDIX  I. 


When  these  are  regarded  it  is  found  that  both  arms  of  the  cross 
show  torsion  as  in  the  hemispherical  field,  though  the  distortion 
due  to  projection  seems  at  first  to  have  turned  the  two  arms  in 
opposite  directions.1 

This  exposition  has  necessarily  been  physiological  and  geometri- 
cal.    The  psychological  interest  in  the  matter  depends  on  the  fact 


E 


K       G 


that  the  perception  of  space  with  the  eye  at  rest  is  profoundly  af- 
fected by  its  experiences  in  or  after  motion,  a  large  group  of  which 
are  received  while  the  eye  is  functioning  in  more  or  less  accord  with 
Listing's  law.  For  a  fuller  account  of  these  psychological  matters, 
see  Ex.  172. 


1  By  an  error  in  drawing,  these  solid  black  arms   are  placed  beside   the 
lines  on  which  they  should  lie. 


APPENDIX   II. 


Some  Simple  Cases  of  the  Mathematical  Horopter. 

THE  mathematical  treatment  of  the  horopter  lies  outside  the  scope 
of  this  work,  but  a  geometrical  description  of  some  special  cases  may 
help  to  make  the  matter  clear.  Any  such  description  must  be  based 
Upon  certain  assumptions  with  regard  to  the  distribution  of  corre- 
sponding points  and  the  movements  of  the  eyes.  It  is  assumed  in 
what  follows,  for  example,  that  corresponding  points  in  both  eyes 
are  so  distributed  that  equal  distances  from  the  foveas  in  the  same 
direction  always  give  corresponding  points,  that  there  is  no  devia- 
tion of  the  retinal  verticals  (cf.  Ex.  209  6),  and  that  in  movements 
with  parallel  lines  of  regard  the  eyes  follow  Listing's  law,  while  in 
convergence,  they  rotate  about  the  line  of  regard  as  observed  in  Ex. 
133.  The  special  cases  taken  for  description  are  those  arising  with 
parallel  lines  of  regard,  and  with  convergence  in  different  positions 
of  the  plane  of  regard. 


THE   HOROPTER  WITH   PARALLEL   LINES   OF   REGARD. 

Imagine  lines  of  direction  drawn  from  all  the  corresponding  points 
in  both  eyes  through  the  respective  crossing  points  of  lines  of  direc- 
tion and  continued  to  infinity.  The  points  seen  single  must  lie  at 
the  intersection  of  corresponding  lines  of  direction  When  the  lines 
of  regard  are  parallel  and  the  eyes  are  unrotated,  all  these  corre- 
sponding lines  are  parallel,  i.e.,  meet  at  infinity,  and  the  horopter 
will,  therefore,  be  a  hemisphere  of  infinite  radius,  or  what  amounts 
to  the  same  thing,  a  plane  at  an  infinite  distance  perpendicular  to 
the  lines  of  regard. 

This  is  true  whether  the  lines  of  regard  are  in  the  primary  or  in 
a  secondary  position,  for  so  long  as  the  eyes  take  their  positions 
according  to  Listing's  law,  no  rotation  of  the  eyes  about  the  linos 

427 


428  '  APPENDIX  II. 

of  regard  is  required,  and  the  corresponding  lines  of  direction  remain 
continuously  parallel.1 

THE    HOROPTER    WHEN    THE    LINES    OF    REGARD    ARE    CONVERGED 
IN   THE    PRIMARY   POSITION   FOR   CONVERGENCE.2 

The  horopter  in  this  case  is  the  major  part  of  a  circle  passing 
through  the  crossing  points  of  the  lines  of  direction  in  the  two  eyes 
and  a  perpendicular  to  the  circle  at  the  fixation  point. 

It  is  not  difficult  to  show  that  this  is  so,  if  use  is  made  of  a  few 
retinal  land  marks.  Imagine  the  eyes  directed  straight  forward 
with  parallel  lines  of  regard.  A  plane  passed  through  both  lines  of 
regard  will  cut  the  retinas  in  their  horizontal  meridians  or  retinal 
horizons.  Planes  passed  through  the  lines  of  regard  perpendicular 
to  the  plane  of  regard  will  cut  the  retinas  in  their  vertical  meri- 
dians or  retinal  verticals.  These  lines  are  the  landmarks  needed. 
In  the  present  case  the  planes  of  the  retinal  horizon  will  coincide 
with  the  plane  of  regard,  and  the  planes  of  the  retinal  verticals  will 
be  perpendicular  to  that  plane  and  intersect  in  a  line  perpendicular 
to  it  at  the  fixation  point. 

In  Fig.  A  the  letters  L  and  E  mark  the  crossing-points  of  the 
lines  of  direction  in  the  two  eyes;  F  is  the  fixation  point;  LF  and 
EF  mark  the  intersection  of  the  planes  of  the  retinal  verticals  with 
the  plane  of  regard  (and  are  thus  also  the  lines  of  regard);  the 
plane  of  the  paper  is  the  plane  of  regard.  The  horopter  is  com- 
posed of  the  circle  FRLP  (except  the  part  EL  between  the  eyes), 
and  the  perpendicular  is  a  perpendicular  to  the  plane  of  the  paper 
at  F.  The  circle  is  known  as  the  "Circle  of  Miiller,"  and  the  per- 
pendicular (less  frequently)  as  the  horopteric  line.8 


1  This  may  seem  to  contradict  the  statement  on  p.  424,  that  such  rotations 
are  present  in  movements  from  one  secondary  position  to  another  ;  but  the  con- 
tradiction is  only  apparent,  for  the  rotation  in  such  cases  is  a  rotation  with 
reference  to  the  condition  of  the  eye  in  its  secondary  starting-point,  and  not  with 
reference  to  its  primary  position.    Even  if  there  were  rotation,  the  statement 
in  the  text  would  hold,  for  since  the  lines  of  regard  are  parallel,  the  degree  of 
rotation  would  be  the  same  in  both  eyes. 

2  The  primary  position  for  convergence  is  that  depressed  position  of  the 
plane  of  regard  in  which  convergence  is  possible  without  rotation  of  the  eyes 
about  the  line  of  regard.    (Cf .  Ex.  133,  p.  126.) 

a  Footnote  1  on  p.  271  states  that  the  Circle  of  Miiller  lies  in  the  plane  of 
regard  when  the  eyes  are  in  the  "  primary  position."  This  is  the  case  only 
when  "  primary  position  "  is  understood  in  the  sense  of  "  primary  position  for 
convergence." 


THE  MATHEMATICAL  HOROPTER. 


429 


It  is  easy  to  show  that  the  points  of  the  circle  will  be  projected  on 
corresponding  points  of  the  retinas.  Both  retinal  images  of  the 
point  P,  for  example,  lie  on  the  retinal  horizons,  and  both  lie  at 
equal  distances  to  the  right  of  the  foveas,  because  the  angle  PLF  is 
equal  to  the  angle  PRF,  both  being  measured  by  half  the  arc  PF. 
Any  point  inside  of  Miiller's  circle,  including  the  part  of  the  circle 
itself  between  L  and  R.  will  give  heteronymous  double  images,  and 
the  points  outside,  homonymous  images.  The  same  will  be  true  of 
any  points,  lying  above  or  below  the  plane  of  regard,  which  lie  also 
inside  or  outside  of  a  cylindrical  surface  erected  on  this  circle  per- 
pendicular to  that  plane.  The  perpendicular  at  F  is  clearly  in  the 


horopter  because  its  images  will  lie  on  the  vertical  meridians  of  both 
eyes,  and  every  point  of  it  will  give  an  image  equally  distant  from 
the  fovea  on  that  meridian  in  each  eye.  Other  perpendiculars  to 
Miiller's  circle,  e.g.,  one  at  P,  contain  but  a  single  point  each  that 
belongs  to  the  horopter  (the  point  in  which  they  touch  the  circle), 
because,  being  unequally  distant  from  the  two  eyes,  all  other  points 
have  images  that  lie  at  different  distances  above  or  below  the  retinal 
horizons. 

This  will  be  clear  from  the  figure  below.  The  image  of  every 
point  on  the  line  MN  must  fall  on  disparate  points  in  L  and  R 
except  that  of  the  point  P,  for  in  every  case  the  angle  in  the  left 
eye  will  be  greater  than  in  the  right.  The  same  will  be  true  of  per- 
pendiculars to  the  portion  of  the  circle  at  the  right  of  the  fixation 


430 


APPENDIX  II. 


point,  except  that  the  angle  would  then  be  greater  in  the  right  than 
in  the  left.1 

When  the  convergence  is  asymmetrical  the  horopter  remains 
exactly  the  same,  the  perpendicular  lying  not  at  the  fixation  point, 
but  in  the  median  plane  of  the  head,  as  it  did  for  symmetrical  con- 
vergence. In  Fig.  B  above,  F'  is  the  asymmetrical  fixation  point. 
The  horopter  is  Miiller's  circle  EF'FL,  with  the  exception  of  the 
part  RL,  and  the  perpendicular  is  at  F.  The  reasons  for  this  can 
easily  be  gathered  from  what  has  already  been  said. 


THE    HOROPTER    WHEN    THE    LINES    OF    REGARD    ARE    CONVERGED 
IN   OTHER   THAN   THE    PRIMARY    POSITION   FOR   CONVERGENCE. 

When  the  eyes  are  converged  symmetrically,  and  the  plane  of  re- 
gard is  higher  than  in  the  case  just  considered  (that  is,  in  the  vast 
majority  of  cases),  convergence  is  attended  by  outward  rotation  of 
the  eyes  about  the  line  of  regard,  the  right  eye  rotating  to  the  right 
and  the  left  eye  to  the  left.  The  horopter  is  consequently  altered  in 
form.  The  horopteric  line  is  no  longer  perpendicular  to  the  plane 
of  regard,  but  inclines  backward  from  the  face  of  the  observer.  The 
reason  for  this  inclination  will  be  clear  if  the  reader  returns  to  the 
planes  passed  through  the  vertical  meridians  of  the  eyes,  and  ima- 
gines their  rotation  outward.  The  horopteric  line  is  the  line  of 
intersection  of  these  planes;  and  as  the  eyes  rotate  outward,  the  line 
of  intersection  rotates  backward  about  the  fixation  point. 

The  rotation  of  the  eyes  also  breaks  up  the  coincidence  of  the 
planes  of  the  retinal  horizons  with  each  other  and  with  the  plane  of 


1  Ex.  210  c  may  seem  to  show  something  different,  but  the  conditions  there 
ire  such  as  to  prevent  the  observation  of  images  doubled  vertically. 


THE  MATHEMATICAL   HOROPTER. 


431 


regard;  a  horopteric  circle  in  that  plane  is  therefore  impossible. 
But  as  the  eyes  rotate,  other  pairs  of  corresponding  planes  (passing 
through  the  same  horizontal  axis  of  the  eye  but  inclined  downward) 
fall  together.  The  planes  that  coincide  are  always  perpendicular  to 
the  inclined  horopteric  line,  and  in  them  the  horopteric  circle  lies. 
It  does  not,  of  course,  cut  the  horopteric  line  in  the  fixation  point. 
This  form  of  the  horopter  is  represented  in  the  following  cut. 

The  plane  of  the  paper  is  the  median  plane  of  the  head.  FA 
represents  the  intersection  of  the  plane  of  regard  with  the  median 
plane.  DE  is  the  inclined  horopteric  line.  AB  is  the  intersection 
of  the  plane  of  the  horopteric  circle  with  the  median  plane.  In 
ordinary  vision  the  rotation  of  the  eyes  is  very  small,  and  the  plane 
of  AB  is  so  little  depressed  below  the  plane  of  regard  as  to  be 
practicably  indistinguishable  from  it. 


E 


When  the  convergence  is  asymmetrical  and  the  plane  of  regard 
other  than  the  primary  one  for  convergence,  the  horopter  becomes 
a  curve  of  double  curvature,  lying  in  the  surface  of  a  circular  cylinder, 
and  too  complicated  for  easy  exposition  in  this  way.  The  following, 
from  Hering's  Beitrdge  zur  Physiologic  (p.  228  f.)  shows  the  rela- 
tion of  this  curve  to  the  forms  of  the  horopter  just  considered:  "  The 
horopter,  made  up  of  the  circle  and  straight  line,  may  be  regarded 
as  a  curve  of  a  single  branch,  which  comes  as  a  straight  line  from 
infinity,  suddenly  makes  a  right-angular  turn,  passes  then  as  a  circle 
through  both  crossing-points  of  lines  of  direction,  and  having  re- 
turned to  the  point  of  inflection,  makes  another  right-angular  turn, 


432  APPENDIX  II. 

and  then  continues,  once  more  as  a  straight  line,  to  infinity  in  a 
direction  opposite  to  that  from  which  it  came.  The  horopter  curve 
at  other  times  also  takes  a  closely  similar  course.  It  comes  from 
infinity  with  very  slight  curvature  on  the  surface  of  the  cylinder  in 
question,  suddenly  makes  a  more  or  less  blunt  turn,  goes  on,  approxi- 
mately in  a  circle,  though  always  with  double  curvature,  through 
both  crossing-points,  returns  thus  nearly  to  the  first  turn,  makes, 
without  touching  it,  another  blunt  turn,  and  runs  once  more  with 
slight  curvature  to  infinity  in  a  direction  opposite  to  that  from  which 
it  came.  The  former  right-angular  turns  with  points  in  contact,  in 
this  case,  are,  as  it  were,  drawn  apart." 

For  a.diagram  showing  the  same  in  graphic  form,  see  Helmholtz's 
Physiologische  Optik,  2te  Aufl.,  p.  861  (p.  714  of  the  first  edition). 


NOTES   AND   SUGGESTIONS 


Ex.  7.  For  systematic  determinations,  use  an  adaptation  of  the 
method  of  minimal  change  explained  in  Exs.  24  and 
237. 

17.  The  insertion  of  the  second  finger  must  be  delayed  till  the 
sensation  in  the  first  finger  has  developed  fully,  but  not 
until  the  physiological  zero  has  shifted. 

ISa.  If  this  experiment  is  to  be  conclusive,  sufficient  time  must 
be  allowed  for  change  in  the  actual  temperature  of  the 
finger  brought  from  the  hotter  or  colder  water. 

23.  In  the  form  here  given  this  experiment  is  of  little  value, 
and  may  well  be  cancelled.  In  order  to  be  comparable 
with  cases  apparently  similar  (Exs.  17  and  143&),  it  is 
necessary  that  the  weight  per  unit  area  of  surface  should 
be  the  same.  If  this  is  so,  the  result  would  probably  be 
reversed.  Care  must  be  taken  to  choose  a  surface  for  ex- 
periment that  will  allow  the  larger  weight  to  touch  over 
its  entire  surface. 

35.  This  experiment  as  described  depends  on  the  determina- 

tions reached  in  Ex.  34  6,  but  this  is  unnecessary.  It 
is  simpler  and  better  to  furnish  the  subject  with  equal 
weights,  one  in  either  hand,  and  to  require  him,  after 
lifting  them  alternately,  to  announce  which  seems  the 
lighter.  This  may  then  be  taken  as  the  "  standard,"  and 
the  other  as  "  the  weight  to  be  compared,"  for  which  the 
2  kg.  weight  is  later  substituted,  and  the  experiment  con- 
tinued as  explained  in  the  text. 

36.  It  should  be  more  distinctly  stated  in  this  case  that  the 

explanation  in  favor  of  innervation  sensations  is  in 
the  nature  of  a  quotation,  and  does  not  represent  the 
writer's  opinion. 

433 


434  NOTES  AND   SUGGESTIONS. 

446.  For  "millimeter  scale"  read  "meter  stick." 

d.  This  experiment  fails  frequently  with  the  operator  seated ; 
better  results  would  possibly  be  reached  by  making  the 
experiment  while  he  is  standing. 

This  experiment  should  be  made  with  unison  forks. 

The  movement  of  the  candle  should  be  of  small  extent  — 

an  inch  or  two  perhaps, 
c.  The  card  must  be  held  as  close  as  possible  to  the  eye. 

116.     This  experiment  will  be  easier  if  tried  with  the  single  eye. 

117a.  The  way  of  finding  the  least  distinguishable  visual  angle 
given  in  this  experiment,  and  the  inferences  from  it  as 
to  the  size  of  the  perceiving  elements  of  the  retina,  are 
discredited,  and  the  experiment  should  be  cancelled. 

The  data  for  calculating  the  visual  angle  given  in  line 
9  should  read,  "about  7  mm.  back  of  the  cornea  and 
15  mm.  in  front  of  the  retina,"  instead  of  as  given. 

P.  119.  Introduction  to  experiments  on  eye  movements.  The  defi- 
nition of  the  field  of  regard  should  be  made  to  read: 
"  The  Field  of  Regard  is  the  extent  of  space  within 
which  the  Point  of  Regard  may  be  moved  without  head 
movements." 

Ex.134.  Unless  the  head  is  fixed  in  this  experiment,  involuntary 
movements  of  it  may  complicate  the  results. 

145c.  The  flickering  observed  may  be  due  to  other  causes  than 
that  suggested,  and  the  experiment  should  therefore  be 
cancelled. 

147.  In  this  experiment  a  greater  blackening  of  the  black  sec- 
tors is  to  be  observed  when  attention  is  so  directed,  as 
well  as  a  brightening  of  the  white.  The  brightness  of 
such  an  object  would  appear  to  be  judged,  when  atten- 
tion is  indifferent,  from  the  brightness  of  its  brightest 
parts. 

I50a.  (Diagram,  p.  151.)  The  arrangement  shown  does  well 
enough  if  the  background  is  white.  If  it  is  black,  gray 
fields  should  take  the  place  of  the  white. 

152a.  Cancel  the  second  paragraph,  with  reference  to  the  supe- 
rior brightness  of  the  yellow  shadow. 


NOTES  AND   SUGGESTIONS.  435 

d.  Second  paragraph,  top  of  p.  159.  The  approach  of  the 
white  card  cuts  off  a  part  of  the  inducing  color,  and  this 
may  account  for  the  effect,  in  which  case  the  experiment 
would  be  of  no  importance  for  the  theory  of  contrast. 
154a.  The  statement  made  at  the  end  of  the  paragraph,  with 
reference  to  the  absence  of  a  dark  halo  surrounding  the 
after-image  of  a  black  square  on  a  white  ground,  is  ap- 
parently an  error.  If  this  is  the  case,  the  whole  of  a 
loses  special  interest  for  the  theory  of  contrast. 

191d.  The  experiments  of  Pierce,  published  since  this  paragraph 
was  written  (Psychological  Review,  5.  1898,  233-253), 
have  demonstrated  that  the  illusion  in  this  figure  is  due 
to  irradiation.  It  should  therefore  be  omitted  here  and 
considered  with  the  other  irradiation  figures  in  Ex.  235. 


INDEX   OF  AUTHOES. 


[  When  the  same  work  has  been  cited  in  the  bibliographies  of  more  than  one 
chapter  the  repetitions  have  been  placed  in  parentheses.] 


ABNEY,  176. 

ABNEY  AND  TESTING,  176. 

ALBERT,  176. 

ARISTOTLE,  21. 

ARONSOHN,  51. 

ARRER,  325. 

AUBERT,    43,    127,    (176),    325, 

(362). 

AUBERT  AND  KAMMLER,  21. 
AUERBACH,  325. 
AYRES,  44. 

BAILEY  AND  NICHOLS,  51. 
BAIN,  24. 
BALDWIN,  325. 
BASTIAN,  44. 
BEAUNIS,  21,  (44),  325. 
BELLARMINOW,  176. 
BENSON,  176. 
YON  BEZOLD,  85,  127,  176,  177, 

325. 

VAN  BIERVLIET,  325. 
BINET,  325. 
BLASCHKO,  21. 
BLASERNA,  88. 
BLIX,  21. 
BLOCK,  21,  44. 
BOURDON,  325. 
BOWDITCII,  325,  (362),  301. 


BOWDITCH    AND    HALL,     127, 

(325). 

BOWDITCH  AND  SOUTHARD,  325. 
BOWDITCH  AND  WARREN,  378. 
BRENTANO,  325. 
BREUER,  44. 
BREWSTER,  325. 
BRODHUN,  177. 
BRONSON,  21. 
BROWN,  44. 
BRUCKE,  85,  177,  326. 
BRUNOT,  326. 
BUDDE,  127,  (326). 
BURMESTER,  326. 
CAMERER,  51. 
CAMPBELL  AND  MUNSTERBERG, 

329. 
CATTELL  AND  FULLERTON,  45, 

(362). 
CHARPENTIER,  44,  85,  127,  177, 

326. 

CHAUVEAU,  177. 
CHEVREUL,  177. 
CORIN,  52. 
CORRADI,  85. 
CROSS  AND  GOODWIN,  86. 
CROSS  AND  MALTBY,  86. 
DELAIJARRK,  44,  127,  177. 


437 


438 


INDEX  OF  AUTHORS. 


DELAGE,  44. 

DELBCEUF,  326,  362. 

DESSOIR,  21,  24. 

DlETERICI  AND  A.  KONIG,  181. 

DIXON,  326. 

DOCQ,  86. 

DONALDSON,  21,  24. 

DONALDSON  AND  HALL,  22. 

DONDERS,  177. 

DOVE,  177. 

DRESSLAR,  326. 

Du  BOIS-REYMOND,  C.,  326. 

Du  BOIS-REYMOND,  R.,  21. 

DVORAK,  128,  326. 

EBBINGHAUS,  177,  178. 

EBERT,  178. 

EINTHOVEN,  326. 

EWALD,  44. 

EXNER,  86,  128,  178,  326. 

FECHNER,  21, 178,  (326). 

FERKIER,  44. 

FESTING  AND  ABNEY,  176. 

FICK,  A.,  128,  178. 

FICK,  A.  E.,  178. 

FICK,  A.  E.  AND  GtiRBER,  128. 

FlLEHNE,  326. 

FISCHER,  O.,  326. 
FISCHER,  R.,  327. 
VON  FLEISCHL,  128,  (327). 
FRANKLIN,    CHRISTINE    LADD, 
178,  327. 

FULLERTON    AND    CATTELL,  45, 

(362). 

FUNKE,  21,  (45). 
GALTON,  379,  398. 
GOLDSCHEIDER,  22,  24,  45. 
GOLDSCHEIDER  AND  SCHMIDT, 

52. 

GOLTZ,  22. 
GOODWIN  AND  CROSS,  s<>. 


GREEFF,  327. 

GRtJTZNER,  327. 

GtJRBER  AND  A.  E.  FlCK,  128. 

GUYE,  327. 

HALL     AND     BOWDITCH,    127, 

(325). 

HALL  AND  DONALDSON,  22. 
HALL  AND  HARTAVELL,  45. 
HALL  AND  MOTORA,  22. 
HARTWELL  AND  HALL,  45. 
HAYCRAFT,  52. 
HELMHOLTZ,  86,  128,  129,  178, 

179,  (327),  (362). 
HENSEN,  86. 
BERING,  22,  129, 179, 180,  (327), 

362,  (408). 
HERMANN,  52,  86. 
HERROUN  AND  YEO,  86. 
HERZEN,  22. 
HESS,  129,  180. 
HEUSE,  129. 
HEYMANS,  327. 
HILLEBRAND,   129,   (180),  327, 

408. 

HOFLER,  327. 
HOLMGREN,  180. 
HOLTZ,  327. 
HOPKINS,  327. 
HOPPE,  22,  327. 
HOWELL  AND  KASTLE,  52. 
HYSLOP,  328. 

JAMES,  23,  (45),  (86),  (328). 
JASTROW,  181,  328,  362, 364,  417. 
JEFFRIES,  181. 
JUDD,  328. 

KAMMLER  AND  AUBERT,  21. 
KAMPFE,  374. 
KASTLE  AND  HOWELL,  52. 
KKPPLKK,  52. 
KESSEL,  86. 


INDEX  OF  AUTHORS. 


439 


KIRSCHMANN,  181,  328,  362,  413. 
KNOX  AND  WATANABE,  328. 
KONIG,  A.,  181. 

KONIG,  A.,  AND  DlETERICI,  181. 

KONIG,  R.,  86. 

KREIDL,  45. 

YON  KRIES,  86,  181,  328. 

KULPE,  362. 

KUNDT,  328. 

LADD,  23. 

LAMANSKY,  129. 

LANGE,  86,  (328). 

LAQUEUR,  129,  (328). 

LASKA,  328. 

LE  CONTE,  129,  328.  /  f 

LEHMANN,  181. 

LEUBA,  362. 

LIPPS,  329. 

LOEB,  23,  45,  329. 

LOMBROSO,  23. 

LOMBROSO    AND    OlTOLENGHI, 

23,  (52). 
LORENZ,  86. 
LOTZE,  23. 
LUFT,  86. 

MACH,  45,  (86),  (129),  329. 
MALTBY  AND  CROSS,  86. 
MANTEGAZZA,  23. 
MARSHALL,  24. 
MARTIUS,  329. 

MARTIUS-MATZDORFF,  329,  406. 
MAXWELL,  129,  182. 
MAYER,  87,  182,  383,  (384). 
MAYERHAUSEN,  329. 
MESSER,  329. 
MEYER,  H.,  182. 
MEYER,  M.,  AND  STUMPF,  380. 

MOTORA  AND  HALL,  22. 
MiJLLER-LYER,  329. 
MftLLER  AND  SCHUMANN,  J~>. 


MtJNSTERBERG,  45,  87,  329. 
MtJNSTERBERG  AND  CAMPBELL, 

329. 

NICHOLS,  E.  L.,  182. 
NICHOLS,  E.  L.,  AND  BAILEY,  51. 
NICHOLS,  H.,  24,  330. 
NOVIZKI  AND  SCHAR—IX,  330. 
OEHRWALL,  52. 
OPPEL,  330. 
OTTOLENGHI    AND    LOMBROSO, 

23,  (52). 
PACE,  182. 
PASSY,  52. 

PEIRCE,  B.  O.,  JR.,  182. 
PEIRCE,  C.  S.,  182. 
PIERCE,  E.,  330. 
PLATEAU,  182,  183. 
POLE,  182. 

PREYER,  23,  87,  (182). 
QUANTZ,  330. 
QUINCKE,  23. 
RAMSEY,  52. 
RAYLEIGH,  87,  182. 
RAYLEIGH  AND  OTHERS,  182. 

RlTTMEYER,  52. 

RIVERS,  330. 

ROGERS,  330. 

ROOD,  129,  183,  330. 

ROUSE,  330. 

RUTHERFORD,  87. 

SACHS,  130. 

SANFORD,  415. 

SAVELIEFF,  52, 

SCHAEFER,  45,  87. 

SCHAPRINGER,  330. 

SCHARWIN  AND  NOVIZKI,  330. 

SCHISCHMANOW,  87. 

SCHMIDT  AND   GOLDSCHEIDER, 

52. 
SCHOX,  330. 


440 


INDEX  OF  AUTHORS. 


SCHUMANN  AND  MULLER,  45. 
SCHUSTER,  183. 
SCHWANER,  23 

SCHWARZ,  130 

SERGI,  23. 

SHAW  AND  H.  C.  WARBEN,  331. 

SHORE,  52. 

SORET,  330. 

SOUTHARD  AND  BOWDITCH,  325. 

STERN,  330. 

STERNBERG,  45. 

STEVENS,  330,  331. 

STRATTON,  331. 

STUMPF,  23,  87,  88. 

STUMPF  AND  MEYER,  380. 

SULLY,  331. 

SZABADFOELDI,  23. 

SZILI,  331. 

TALBOT,  183. 

TAYLOR,  88. 

THIERY,  331. 

THOMPSON,  S.  P.,  88,  331. 

TITCHENER,  183. 

TREITEL,  130. 

TSCHERNING,  130. 

TUMLIRZ,  130. 


TYNDALL,  88. 

UHTHOFF,  130. 

URBANTSCHITSCH,  88. 

VIERORDT,  23,  46. 

VON  VINTSCHGAU,  52. 

WALLENBERG,  331. 

WALLER,  46,  331. 

WARREN,  J.  W.,  AND  BOW- 
DITCH,  378. 

WARREN,  H.  C.,  AND  SHAW,  331. 

WASHBURN,  MARGARET  F.,  331, 
364. 

WATANABE  AND  KNOX,  328. 

WEBER,  23,  (46). 

WHEATSTONE,  331,  332. 

YON  WITTICH,  23. 

WLASSAK,  46. 

WOLF,  130. 

WOOD,  331. 

WUNDT,  23,  (46),  (52),  88,  130, 
183,  (332),  (362). 

YEO  AND  HERROUN,  86. 

ZAHN,  88. 

ZEIIFUSS,  130. 

ZOLLNER,  332. 

ZWAARDEMAKER,  53,  373. 


INDEX   OF  SUBJECTS. 


Accommodation,  90  ff . ;  connec- 
tion of,  with  iris  movements, 
99;  line  of,  93;  mechanism  of, 
93 ;  perception  of  depth  by,  203 ; 
range  of,  92 ;  Schemer's  experi- 
ment for,  90. 

Active  touch,  5. 

Acuteness  of  vision,  106. 

Adjustment  of  motor  discharge, 
27. 

^Ssthesiometric  compasses,  4,  364. 

After-images :  auditory,  57 ;  effect 
of  eye-movements  on,  115 ;  false 
location  of,  200 ;  negative,  112 ; 
of  motion,  116,  302 ;  of  tempera- 
ture, 8,  11 ;  of  touch,  6 ;  on  dark 
and  on  light  backgrounds,  114 ; 
on  ground  of  the  same  color, 
112 ;  positive,  113  f . ;  projection 
of,  112,  186j  seat  of,  115;  se- 
quence of  colors  in,  114 ;  visual, 
112  ff. 

Algometer,  365. 

Anaglyphs,  298. 

Analysis  of  groups  of  simultaneous 
tones,  72 ;  assisted  by  difference 
in  location,  82. 

Antirrheoscope,  118,  417. 

Apparatus,  363  ff. ;  alternate  forms 
of,  416  ff ;  minimal  list  of,  419; 
for  dermal  senses,  363  ff. ;  for 
hearing,  373  ff. ;  for  kinaesthetic 
and  static  senses,  366  ff. ;  for 
taste  and  smell,  369  ff. ;  for 


vision,  386  ff. ;  for  Weber's  law 
and  the  psychophysic  methods, 
412  ff. 

Apparent  movement  of  an  im- 
movable member,  30 ;  of  a  single 
point  of  light,  309,  of  objects 
viewed  with  eyes  in  constrained 
positions,  310  (see  also  After- 
images of  movement) ;  of  pitch 
in  successive  chords,  77. 

Appendices,  421,  427. 

Apperceptive  completion  of  move- 
ments, 312. 

Appunn's  lamella,  381. 

Aristotle's  experiment,  2. 

Associate  points,  266. 

Astigmatism,  95  f. 

Attention,  effect  of,  on  apparent 
size,  197 ;  on  successive  chords, 
77. 

Auditory  after-images,  57 ;  fatigue, 
55  f. ;  inertia,  57. 

Autokinetic  sensations,  309. 

Average  error,  359  note. 

Balances,  416. 

Batteries,  416. 

Beats,  66  ff . ;  binaural,  82;  of  oc- 
taves, 67;  rate  giving  greatest 
roughness,  67;  with  very  faint 
tones,  67. 

Bergmann's  experiment,  107. 

Von  Bezold's  experiment,  97 ;  fig- 
ure, 215. 


441 


442 


INDEX  OF  SUBJECTS. 


Bibliographies:  20  ff.,  43  ff.,  51  ff., 
85  ff.,  127  ff.,  176  ff.,  325  ff.,  362. 

Bilateral  asymmetries  of  position 
and  motion,  34  ff . ;  bilateral 
movements  with  arms  in  unlike 
positions,  28. 

Binaural  audition,  81  ff . ;  beats,  82 ; 
-  differences  in  pitch,  62. 

Binocular  after-images,  175;  color- 
mixer,  164, 399  ff . ;  color-mixing, 
173 ;  combination  of  slightly  dif- 
ferent diagrams,  291  f .,  of  stereo- 
scopic diagrams  with  free  eyes, 
276;  contrast,  174;  direction, 
263 ;  field  of  vision  and  field  of 
regard,  261 ;  fields  indistinguish- 
able, 262;  judgments  of  depth, 
281 ;  line  of  regard,  263 ;  locali- 
zation, as  effected  by  changes  in 
convergence  and  in  retinal  im- 
ages, 285 ;  localization  vs.  monoc- 
ular, 282  ff. ;  perception  of  light 
and  color,  168  ff.,  of  relief,  275, 
294  ff.,  of  space,  260  ff. ;  strobo- 
scope,  296,  disks  for,  396 ;  vision, 
unusual  eye  movements  to  se- 
cure, 289  ff. 

Black,  138. 

Blending  of  tones,  71. 

Blind  spot,  102  ff. ;  filling  out  of, 
103. 

Bottle  whistles,  384  f . 

Bradley's  pseudoptics,  213,  419. 

Briicke's  experiment,  146. 

Cadences,  80. 

Campimeter,  387  f. 

Centre  of  rotation  of  the  eye,  119. 

Chords,  apparent  movement  in  suc- 
cessive, 77 ;  consonant  and  disso- 
nant, 79 ;  lowest  tone  in,  fixes 
apparent  pitch,  72;  major  and 
minor,  79. 

Chromatic  aberration,  95,  97,  203, 
208,  297. 


Chromatokinopsia,  318. 

Chrome-alum  solution,  386  f. 

Circle  of  Miiller,  271,  428. 

Clamps,  standards,  etc.,  415 ;  swivel 
and  ball-joint  clamps,  415. 

Cog-wheel  figure,  315. 

Cold  spots,  7. 

Color,  131  ff. ;  complementary,  149; 
induction  of  a  like  color,  165  •, 
intensity,  132,  changes  in,  140  f . ; 
mixed  colors,  147 ;  saturation, 
132,  changes  in,  137 ;  wheel,  144 ; 
zones  of  the  eye,  135  f . ;  see  also 
Contrast. 

Color-blindness,  133;  in  the  periph- 
eral field,  135. 

Color-mixer,  391  ff.,  reflection,  150, 
399;  color-mixing,  147  ff.,  by 
double  refraction,  151,  Lam- 
bert's method  of,  150,  rate  of  ro- 
tation required  for,  144,  spectral 
colors,  151. 

Color-tone,  132;  changes  in,  137. 

Colored  gelatine,  glass,  and  papers, 
386 ;  colored  shadows,  155,  162. 

Combination  tones,  69. 

Complementary  colors,  149. 

Compound  tones,  73;  analysis  of, 
74  ff. 

Concave  mirror,  404. 

Consonant  and  dissonant  intervals, 
78 ;  chords,  79. 

Constant  errors,  347  f .,  352  f.,  355  f., 
360. 

Contour:  illusions, 242 ;  prevalence 
and  rivalry,  169,  171. 

Contrast,  153  ff. ;  conditions  influ- 
encing, 159 ;  disks  for,  158,  396 ; 
in  curvature,  240;  in  spatial 
perception,  238  ff. ;  mirror  con- 
trasts, 155 ;  mixed  contrasts,  155 
ff. ;  simultaneous,  162  ff. ;  suc- 
cessive, 154 ;  of  temperature  sen- 
sations, 9. 

Contrastive  illusions,  238. 


INDEX  OF  SUBJECTS. 


443 


Convergence  of  the  lines  of  regard, 
localization  by,  285 ;  rotation  of 
the  eyes  in,  125. 

Corresponding  points,  266. 

Cyclopean  eye,  263. 

Dark  box,  389. 

Deckpunkte,  266. 

Dermal  senses,  1  ff. 

Deviation  of  retinal  vertical,  268. 

Diagrams  for  stereoscope,  406 ;  for 
"  fluttering  heart "  experiment, 
411. 

Difference  tones,  69. 

Direction:  binocular,  263;  monocu- 
lar, 186  ff . ;  of  sounds,  82. 

Discriminative  sensibility  for  com- 
pass points,  4;  for  brightness, 
138;  for  intensity  of  sound,  55; 
lifted  weights,  26 ;  for  odors,  50 ; 
for  pitch,  63;  for  pressure,  14; 
for  taste,  48;  for  temperature, 
11 ;  for  visual  extents,  with  eyes 
at  rest,  194,  with  movement  of 
the  eyes,  196,  with  the  periphery 
of  the  retina,  106. 

Disks,  for  color-mixer,  392  ff . ;  for 
stereoscopy  with  moving  fig- 
ures, 295,  396;  for  stroboscopy, 
296,  396  ff.;  for  Weber's  law, 
335  f .,  412  f . ;  special  phenomena 
of  rotating,  167. 

Disparate  points,  267. 

Dissociate  points,  267. 

Divergence  of  lines  of  regard,  289. 

Dizziness,  sensations  of,  42  ff. 

Donder's  law,  120. 

Doppelpunkte,  267. 

Double  images ;  conditions  that 
help  and  hinder  the  seeing  of, 
291  ff . ;  homonymous  and  heter- 
onymous,  264 ;  location  of,  272 ; 
vertical  double  images,  292. 

Double  refraction,  color  mixing  by, 
151,  prism  for,  399. 


Drawing  instruments,  416. 
Dvorak's  apparatus  for  binocular 
stroboscopy,  417. 

Eccentric  projection  of  touches,  2. 

Effort,  feeling  of,  30. 

Einthoven's  experiment,  297. 

Electrical  stimulation  of  the  eye, 
109 ;  of  the  taste  organs,  49. 

Electrodes,  370,  388. 

Entoptic  phenomena,  98  ff. 

Equal  weights  of  unequal  size: 
pressure,  13,  365 ;  lifted  weights, 
26,  366. 

Equivocal  figures:  figures  with 
equivocal  perspective,  255; 
plane  figures,  254 ;  three  dimen- 
sional figures,  259.  Equivocal 
movement  depending  on  equiv- 
ocal relief,  321. 

Essence  of  clove,  370. 

Ether  spray,  364. 

Eye :  entoptic  phenomena,  98  ff . ; 
interocular  distance,  260;  lag- 
ging of,  when  head  is  turned, 
201 ;  tendency  of  the  eye  to  fol- 
low lines  and  contours,  214,  250. 

Eye-movements,  119  ff.,  421  ff. ;  and 
the  geometrical  illusions,  253; 
associated  movements,  120 ;  em- 
pirical determination  of  actual 
movements,  124;  in  binocular 
perception  of  relief,  287,  292 ;  in 
dizziness,  42 ;  involuntary,  127 ; 
reflex,  120;  rotation,  defined, 
119,  in  convergence,  125;  un- 
usual, in  favor  of  binocular 
vision,  289  ff. ;  with  parallel  lines 
of  regard,  120. 

Eye  positions,  effect  of,  on  orienta- 
tion, 37  f . 

Fatigue,  auditory,  55  f . ;  of  temper- 
ature organs,  7, 10  ;  olfactory,  50 ; 
retinal,  110. 


444 


INDEX   OF  SUBJECTS. 


Fechner's  colors,  143,  disks  for,  395 ; 
fundamental  table,  351;  para- 
doxical experiment,  169;  side- 
window  experiment,  174.  ' 

Feeling  of  effort,  30. 

Ferrier's  experiment,  30. 

Field  of  regard,  119,  421;  hemi- 
spherical, 421;  plane,  425;  of 
vision,  119. 

Figure  of  Mellinghoff  and  Loeb, 
of  Muller-Lyer,  of  Zollner,  etc. 
See  Geometrical  illusions. 

Filled  and  open  spaces  perceived 
by  touch,  5;  by  kinsesthetic 
senses,  33;  by  eye,  233. 

Fixation:  in  darkness,  200;  lines, 
293 ;  Yon  Fleischl's  experiment, 
302. 

"  Fluttering  heart,"  318. 

Fovea,  100. 

Fullerton  and  CattelPs  table,  354. 

Galton  bar,  194,  402;  form  used 
in  demonstrating  psychophysic 
methods,  414. 

General  apparatus,  415  f . 

Geometrical  illusions,  212  ff . ;  con- 
centric circles,  245 ;  contrastive 
illusions,  238 ;  contour  illusions, 
242 ;  convergent  line  figures,  224 
ff . ;  divided  triangle,  250 ;  dumb- 
bell figures,  241 ;  effect  of  adja- 
cent figures,  251,  of  prominent 
lines,  214,  250;  figures  of  Mel- 
linghoff and  Loeb,  241 ;  Bering's 
figure,  226;  illusions  affecting 
the  sides  of  angles,  247,  in  an- 
gles, 218,  in  distances,  depend- 
ing on  their  direction  in  the 
field  of  vision,  235,  of  conflu- 
ence, 232  note,  of  interrupted 
extent,  233,  of  lines  lying  nearly 
in  the  plane  of  regard,  252,  of 
"  solid  "  and  "  leaded  "  type, 
253 ;  Laska's  figure,  251 ;  Lipp's 


figure,  249;  Miiller-Lyer's  fig- 
ure, 229;  overestimation  of  ex- 
tents in  the  upper  part  of  the 
field,  237;  perspective  figures, 
215  ff. ;  Pisco's  figure,  223;  Pog- 
gendorff's  figure,  227 ;  ring  seg- 
ments, 244 ;  trapezoids,  225 ; 
verticals,  235,  249 ;  Wundt's'  fig- 
ure, 227;  Zollner's  figure,  219, 
variants  of,  222  f . 

Geometrical  illusions  with  un- 
moved eyes,  253. 

Graduates,  416. 

Gravity  sense,  39. 

Gray  papers,  386. 

Hairs  as  organs  of  touch,  17. 

Haploscope,  404  f. 

Harmonical,  385. 

Head  rest,  387. 

Hearing,  sensations  of,  54  ff.  See 
also  Analysis,  Beats,  Binaural, 
Chords,  Pitch,  Sound,  Tone. 

Hering's  binocular  method  for  si- 
multaneous contrast,  164;  ex- 
periment of  the  falling  ball, 
287:  figure,  226;  form  of  Zoll- 
ner figure,  222:  halo  (Lichthof), 
161. 

Holmgren's  test  for  color-blind- 
ness, 133. 

Horopter,  269,  427  ff. 

Hot  and  cold  spots,  7. 

Identical  points,  266. 

Idio-retinal  light,  109. 

Illusions,  6,  30,  32,  33,  37,  40,  42, 
192,  197  note,  205,  206,  207,  310; 
of  confluence,  232  note,  242.  See 
also  Equivocal  figures  and  Geo- 
metrical illusions. 

Illusory  movement,  30,  32,  306  ff . 

Indirect  vision.  See  Peripheral 
vision. 

Induction  coil,  390. 


INDEX  OF  SUBJECTS. 


445 


Imiervation  sense,  25,  28  ff. 

Intensity  of  sensation,  in  relation 
to  extent  of  area  stimulated, 
color,  142,  pressure,  13,  taste, 
48,  temperature,  10;  of  high 
tones,  60  f. ;  of  simultaneous 
tones,  mutually  affected,  78. 

Intercranial  location  of  sounds, 
83  ff.,  of  tones,  62. 

Intermittence  of  faint  stimuli: 
sounds,  55 ;  gray  rings,  140. 

Intermittent  stimulation  of  the 
retina,  105,  143. 

Interocular  distance,  260. 

Interrupted  extents.  See  Filled 
and  open  space. 

Intervals,  consonant  and  disso- 
nant, 78;  recognition  of  musi- 
cal, 65. 

Inverted  picture  and  vision  with 
inverted  head,  210. 

Involuntary  movements  of  the 
eyes,  127. 

Irradiation,  336  ff . ;  positive,  336  f . ; 
negative,  338  f. 

Joint  sensations,  31  ff . ;  joint- 
sense  board,  31,  367 ;  movement 
of  last  joint  of  finger,  30,  32. 

Kernflache,  271  note. 

Kernpunkt,  271. 

Kinaesthetic  and  static  senses,  25  ff . 

Kraftsinn,  26. 

Krypteon,  403. 

Lambert's  color-mixer,  150,  399. 

Laska's  figure,  251. 

Le  Cat's  experiment,  185. 

Lichthof,  161. 

Light  and  color,  sensations  of,  131  ff . 

Limen,  346;  average  limen,  346; 
probable  limen,  351. 

Lines  of  direction,  92,  crossing 
point  of,  92 ;  lines  that  appear 
straight  in  indirect  vision,  189 


ff. ;  lines  of  regard,  divergence 
of,  289;  perception  of  the  posi- 
tion and  movement  of,  198  ff., 
in  normal  and  forced  move- 
ments, 199,  primary  and  second- 
ary positions  of,  119. 

Lipp's  figure,  249. 

Listing's  law,  121  ff.,  421  ff. 

Local  signs,  1. 

Locations :  in  the  indirect  field  of 
regard,  202;  of  beats,  68;  of 
continuous  tones,  83 ;  of  differ- 
ence tones,  71;  of  movements, 
32;  of  pressures,  16;  of  single 
and  double  images,  272;  of 
sounds,  81  ff. ;  of  touches,  1  f . 

Luster,  binocular,  172 ;  monocular, 
173. 

Macula  lutea,  105. 
Mariotte's  experiment,  102. 
Masks,  404. 
Mean  variation,  346. 
Mechanical  stimulation  of  temper- 
ature spots,  8 ;  of  the  retina,  108. 
Medallions,  404. 
Method  of  average  error,  357  ff . ; 

of  minimal  change,  14  f .,  344  ff . ; 

of  right  and  wrong  cases,  349  ff .  ; 

classical  form,  350;    simplified 

form,  353. 
Metronome,  398. 
Meyer's  experiment,  157. 
Minimal  list  of  apparatus,  419. 
Minimal  pressure,   13;  odors,  49; 

sounds,  54;  tastes,  48. 
Minimum  visibile,  106. 
Monocular    perception    of    depth, 

203  if.;   of  direction,  186,  188; 

of  space,  185  ff . ;  vs.  binocular, 

282  ff. 
Motion,  perception  of,  on  the  skin, 

3,  32 ;  relativity  of,  307 ;  visual 

perception  of,  301  ff.    See  also 

Apparent  movement. 


446 


INDEX   OF  SUBJECTS. 


Movements :  of  the  eyes  (see  Eyes) ; 
with  partially  contracted  mus- 
cles, 28  f.,  202  f. 

Miiller-Lyer's  figure,  229;  variants 
of,  230  f . 

Muller's  circle,  271,  428. 

Miinsterberg- Jastrow  phenomenon, 
167. 

MUSCSB  volitantes,  98. 

Muscle  sense,  26. 

Negative  after-images,  112 ;  pres- 
sure, 17. 
Noise,  58. 
Notes,  433  ff. 

Odors,  combination  of,  51 ;  mini- 
mal, 49.  See  also  Smell. 

Olfactometer,  50,  371  ff . 

Organs  for  perception  of  gravity, 
38;  of  rotation,  40;  of  taste,  47. 

Orientation,  sensations  of,  36  ff. 
Otolith  sense,  38. 

Outward  reference  of  touch,  1, 2 ;  of 
visual  perceptions,  185. 

Pain,  19. 

Parallax  of  indirect  vision,  187. 

Partial  tones,  73  ff . 

Passive  movement  at  the  elbow,  31. 

Pendulum  for  carrying  a  small 
tuning-fork,  377  ff. ;  pendulum 
metronome,  398;  sound  pendu- 
lum, 373  ff. 

Perception  of  depth  by  means  of 
accommodation,  203;  by  inter-, 
vening  objects,  204;  by  means 
of  shadows,  206 ;  influenced  by 
other  perceptions,  210;  in  in- 
verted pictures  and  with  in- 
verted head,  210. 

Perception  of  flickering,  168. 

Perception  of  motion,  300  ff. ;  in 
indirect  vision,  305. 

Perception  of  position  and  move- 


ments of  the  eyes,  198  ff . ;  in 
normal  and  forced  movements, 
199 ;  fixation  in  complete  dark- 
ness, 200 ;  false  location  of  after- 
images, 200. 

Perception  of  size  of  known  ob- 
jects, 196. 

Perception  of  space  and  motion, 
184  ff. 

Perceptive  inference  of  depth,  204 
ff. ;  of  form,  255  ff. ;  of  motion, 
311 ;  of  size,  215  ff . 

Perimeter,  388. 

Peripheral  vision:  color-blindness 
in,  135;  discriminative  sensi- 
bility in,  106 ;  illusions  of  form 
in,  192 ;  perception  of  flickering 
in,  168 ;  of  light  in,  134 ;  of  mo- 
tion in,  305. 

Perspective  figures,  215;  perspec- 
tive interpretation  of  plane  fig- 
ures, 216. 

Phosphenes,  108. 

Photographic  shutter,  391. 

Physiological  zero,  9. 

Pisco's  form  of  the  Zollner  figure, 
222. 

Piston  whistles,  384. 

Pitch,  apparent,  affected  by  qual- 
ity of  tones,  64 ;  emotional  effect 
of,  62 ;  number  of  vibrations 
necessary  for,  64;  pitch  dis- 
tances, 65 ;  recognition  and  dis- 
crimination of,  62  f. 

Poggendorff's  figure,  227. 

Positive  after-images,  113;  of  mo- 
tion, 302  ff. 

Pressure,  sensations,  12  ff. ;  active, 
refinement  of,  17 ;  balance  for 
testing,  417  f . ;  discriminative 
sensibility  for,  14 ;  intensity  and 
stimulated  area  in,  13 ;  location 
of,  16  f. ;  minimal  pressure,  13; 
negative,  17 ;  pressure  points, 
12;  temperature  and,  16. 


INDEX   OF  SUBJECTS. 


447 


Prevalence  of  contours,  109,  171. 

Primary  position  of  eyes  and  lines 
of  regard,  119;  for  convergence, 
428. 

Prisms,  387 ;  double  refracting,  399. 

Probable  error  of  a  single  observa- 
tion, 359  note ;  of  the  mean,  360 
note. 

Pseudoscope,  280,  411. 

Psychophysic  methods,  341 '  ff . ; 
method  of  average  error,  357 
ff . ;  method  of  minimal  change, 
14  f.,  344  ff. ;  method  of  right 
and  wrong  cases,  349  ff. 

Psychophysic  series,  340,  413. 

Purkinje's  dizziness,  42;  images, 
93  ;  phenomenon,  142  ;  vessel 
figures,  99. 

Quality  of  tones,  76. 

Ragona  Scina's  experiment,  155 ; 
apparatus  for,  399. 

Rate  of  greatest  roughness  of 
beats,  67;  of  rotation  required 
for  blending  colors,  144. 

Relief,  binocular,  275,  294  ff . ;  mo- 
nocular, 203  ff . 

Resonance  bottles,  383. 

Resonators,  384. 

Retinal  adaptation,  111 ;  blood-ves- 
sels, 99  f.  ;  circulation,  101; 
fatigue,  110  ;  image,  89,  and 
perception  of  size,  194,  196,  of 
size  and  distance,  198;  light, 
109;  oscillation,  168;  rivalry, 
170  f. ;  shadows,  185;  vertical, 
deviation  of,  268. 

Rivalry  of  visual  fields,  170  f . 

Rods,  clamps,  and  couplers,  415. 

Rotating  drum,  417. 

Rotation,  sensations  of,  40  ff. 

Rotation  table,  368  f . 

Rubber  hammer,  379. 

Saturation  (see  Color) ;  disks,  395. 


Schemer's  experiment,  90. 

Secondary  positions  of  eyes  and 
lines  of  regard,  119;  straight 
lines  viewed  in,  193. 

Sensations  from  lifted  weights,  26 ; 
from  the  joints,  31  ff. ;  from  the 
tendons,  25,  34 ;  of  contact,  1, 13 ; 
of  double  contact,  3;  of  gravity, 
38 ;  of  hearing,  54  ff . ;  of  innerva- 
tion,  25 ;  of  light  and  color,  131 
ff. ;  of  motion  on  the  skin,  3;  of 
movement  of  members,  31  ff. ; 
of  orientation,  36  ff . ;  of  pain, 
19 ;  of  pressure,  12 ;  of  progres- 
sive motion,  43;  of  resistance, 
33  f. ;  of  rotation,  40  ff. ;  of 
smell,  49  ff . ;  of  taste,  47  ff . ;  of 
temperature,  7  ff . ;  of  tickle,  18 ; 
of  traction,  17. 

Sensibility:  discriminative  (see  Dis- 
criminative) ;  of  different  bod- 
ily regions  to  pressure,  17;  to 
temperature,  12;  to  tickle,  19. 

Sensory  circles,  4. 

Sequence  of  colors  in  after-images, 
114. 

Sighting  lines,  187 ;  common  point 
of,  187  note. 

Silence,  59. 

Similarity,  321. 

Single  and  double  images,  264. 

Smell,  sensations  of,  49  ff. 

Snapper  sounder,  385. 

Solutions  for  testing  taste  and 
smell,  370  f. 

Sonometer,  383. 

Sound,  after-images  of,  58;  mini- 
mal, 54 ;  sound  pendulum,  373 ; 
see  also  Auditory,  Hearing, 
Pitch,  and  Tone. 

Space,  perception  of,  184  ff . ;  spatial 
quality  of  tones,  61. 

Space  error,  347. 

Specific  energy  of  nerves,  8  note ; 
of  temperature  spots,  8. 


448 


INDEX  OF  SUBJECTS. 


Spectroscope,  398. 

Spiral  disk,  394. 

Standards,  rods,  clamps,  and  coup- 
lers, 415. 

Steel  cylinders  for  highest  audible 
tone,  381. 

Stereoscope,404 ;  Wheatstone's,406. 

Stereoscopic  diagrams,  406;  effect 
of  slight  differences  in,  192. 

Stereoscopy,  294  ff . ;  hy  chromatic 
aberration,  297;  by  differences 
in  color,  298;  with  binocular 
stroboscope,  296  ;  with  free 
eyes,  276 ;  with  moving  figures, 
294. 

Stevens's  figure,  300. 

Strobic  circles,  313  ff . 

Stroboscope,  binocular,  296,  396; 
monocular,  311,  397. 

Suggestion  blocks,  366. 

Summation  in  tickle  sensations,  19. 

Surface  of  single  vision,  271. 

Symmetry,  323. 

Table  for  sound  pendulum,  375 ; 

for  Galton  whistle,  380  f . ;  for 
method  of  right  and  wrong 
cases,  351,  354. 

Talbot-Plateau  law,  146. 

Taste,  sensations  of,  47  ff . ;  confu- 
sion of,  with  smells,  47 ;  electri- 
cal, 49;  minimal,  48;  solutions 
for  testing,  370. 

Telegraph  snapper,  385. 

Telestereoscope,  278. 

Temperature,  sensations  of,  7  ff . ; 
and  pressure,  16 ;  chemical  and 
mechanical  stimulation  of,  8; 
in  double  contacts,  3;  physio- 
logical zero,  9  ;  temperature 
points,  7  ;  temperature-point 
seekers,  364. 

Tendon  sense,  34. 

Thermometers,  364  f. 

Thompson's  strobic  circles,  313  ff. 


Tickle,  18. 

Tilt-board,  39,  368. 

Timbre,  76. 

Time  error,  347;  time  discrimina- 
tions by  touch,  5 ;  time  relations 
in  music,  80. 

Tones:  beat  tones,  69  note;  char- 
acteristics of  high  and  low, 
60;  highest,  59;  irregularly  va- 
riable, 66;  lowest  tones,  59; 
mutual  effect  of,  66,  78 ;  simul- 
taneous tones,  66  ff.,  difference 
in  location  assists  analysis  of, 
82 ;  single  and  successive,  59  ff . ; 
unison,  heard  with  two  ears,  81. 

Touch,  1  ff . ;  co-ordination  of  vision 
and  touch,  203;  eccentric  pro- 
jection of,  2;  perception  of  vi- 
bration by,  5. 

Tuning-forks,  381  ff. 

Vibration,  perception  of  by  touch, 
5. 

Vision,  89  ff . ;  binocular,  168  ff .,  260 
ff. ;  co-ordination  of  touch  and, 
203;  influence  of  experience  in, 
166,  204  ff.,  215  ff.,  255  ff.,  311; 
light  and  color,  131  ff . ;  motion, 
300 ;  space,  184  ff . ;  with  inverted 
head,  210. 

Visirlinien,  see  Sighting  lines,  187. 

Visual  angle,  194  note;  similarity 
and  symmetry,  321  ff. 

Waller's  experiment,  207. 
Wavering  of  a  single  light  point, 

308;  of  objects  on  long  fixation, 

308. 
Weber's  law,  333  ff . ;  demonstra- 

tional  experiments  for,  334  ff. ; 

disks  for,  335  f.,  412  f. ;  in  the 

classification  of  stimuli,  340 ;  in 

irradiation,  336 ;  transparencies 

for,  334. 
Weber's  sensory  circles,  4. 


INDEX   OF  SUBJECTS. 


449 


Weights,  equal,  of  unequal  size, 
365  f . ;  for  discriminative  sensi- 
bility, 365  f . ;  for  minimal  pres- 
sure, 365 ;  large,  for  lifting,  366 ; 
weighted  envelopes,  413. 

Wheatstone  stereoscope,  406. 

White,  138. 

Wooden  cylinders  of  equal  weight, 
365. 

Worsteds  for  Holmgren's  color- 
blindness test,  398. 


Wundt-Lamansky  law,  124 ; 
Wundt's  figure,  227. 

Yellow  spot,  105. 

Zoetrope,  311. 

Zollner's  anorthoscopic  illusions, 
310;  figure,  219,  influences  af- 
fecting the  extent  of  illusion  in, 
221,  variants  of,  222. 


V     OF  THE 

UNIVERSITY 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 


This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


6      M1 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


•?MI  .:    *tt 


